Genesis 12:2

"and you will be a blessing". The [rights to give] blessings are given into your hand. Until now, [the rights to give blessings] were in my hands. I blessed Adam, Noah, and You. From now on, you will bless whoever you want. Another understanding [of the idea behind "and you will be a blessing"]: "and I will make you into a great nation" [refers] to that which is said [in the Shmoneh Esreh prayer] "the God of Abraham". "and I will bless you" [refers] to that which is said [in the Shmoneh Esreh prayer] "the God of Isaac". "and I will make your name great" [refers] to that which is said [in the Shmoneh Esreh prayer] "the God of Jacob". [You might think that the first paragraph of the Shmoneh Esreh] should be concluded [with a blessing formulated to include mention of] all [of the forefathers], [but really the Torah] says "and YOU will be a blessing", [meaning] [the nation of Israel] will conclude [first paragraph of the Shmoneh Esreh] with you, but not with them (i.e. the other forefathers).

And I will make thee a great people, and will bless thee, and magnify thy name, and thou shalt be blessed.

AND BE THOU A BLESSING. You will be the blessing by whom people will be blessed, saying, “G-d make thee as Abraham.” To this He added that all families of the earth will cite him in blessing, not just the people of his country alone. It may be that the expression, And in thee shall all the families of the earth be blessed, means that they will all be blessed on his account.
Now this portion of Scripture is not completely elucidated. What reason was there that the Holy One, blessed be He, should say to Abraham, “Leave your country, and I will do you good in a completely unprecedented measure,” without first stating that Abraham worshipped G-d or that he was a righteous man, [and] perfect? Or it should state as a reason for his leaving the country that the very journey to another land constituted an act of seeking the nearness of G-d. The custom of Scripture is to state, “Walk before Me, and hearken to My voice, and I will do good unto you,” as is the case with David and Solomon,, 3:13-14. as well as throughout the Torah: If ye walk in My statute; And it shall come to pass, if thou shalt hearken diligently unto the voice of the Eternal thy G-d. And in the case of Isaac, it says, For My servant Abraham’s sake. But there is no reason for G-d to promise [Abraham a reward merely] for his leaving the country.
However, the reason [for G-d’s promising Abraham this reward] is that the people of Ur of the Chaldees did him much evil on account of his belief in the Holy One, blessed be He, and he fled from them to go to the land of Canaan, tarrying for a time at Haran, whereupon the Eternal told him to leave these places as well and to fulfill his original intention that his worship be dedicated to Him alone and that he call upon people [for the worhip of] the Name of the Eternal in the Chosen Land. There He would make his name great, and these nations would bless themselves by him, not as they treated him in Ur of the Chaldees, where they abused and cursed him, put him in prison or in the fiery furnace. He further told Abraham that He will bless those who bless him, and if some individual will curse him, he will be cursed in turn.
This then is the meaning of this portion of Scripture. The Torah, however, did not want to deal at length with the opinions of idol worshippers and explain the matter between him and the Chaldeans in the subject of faith, just as it dealt briefly with the matter of the generation of Enosh, Note 280. and their thesis concerning the idol-worship which they instituted.

והיה ברכה, a true blessing by G’d is when G’d rejoices in our deeds and actions. Our sages (Berachot 7) illustrate this when they quote a conversation between the High Priest Rabbi Yishmael ben Elisha who, while offering incense on Yom Kippur, had a vision of the angel Katriel sitting on the throne of G’d. The latter said to him: “Yishmael, My son, bless Me.” He said to him: (in the words attributed as being G’d’s own ‘prayer’) “may it be Your will that Your mercy will subdue Your anger and may Your mercy exile Your justified attribute to punish Your people for their sins, and may Your mercy prevail so that You deal with Your children by applying the attribute of Mercy.” When G’d’s representative, the angel Katriel heard this, he touched the High Priest on the head, which the latter took as a sign that the blessing uttered by an inferior creature for a superior should not be dismissed as worthless.” [I have stuck more closely to the text of the Talmud than did the author here. Ed.] G’d here blessed Avram, with becoming the one who would preach monotheism including awareness of the benevolence of G’d which He extends to all of His creatures.

ואעשך לגוי גדול, even though your wife is barren, I will cure her and in this country she will bear a child.

ואברך, I will give you additional goodness, such as material wealth, possessions and honour, as we read in 13,2 “Avram was very rich in cattle, silver, and gold.”

ואגדלה שמך, your name will be known throughout many nations due to how blessed you are, due to your good deeds, and the success of your undertakings. Examples were Avram’s defeat of the four most powerful kings on earth at that time, when he freed Lot and the King of Sodom. The Canaanites (בני חת) said to him when he wanted to buy a burial plot for Sarah “you are a prince in our midst” (Genesis 23,6). We find a similar statement which G’d made to David in Chronicles I 17,8 ועשיתי לך שם בשם הגדולים אשר בארץ, “I will make you as renowned as the greatest men on earth.” This is part of the good that G’d does for people.

והיה ברכה, the construction והיה is similar to the future tense (not imperative) תהיה, i.e. “you will become.” [the author, presumably, finds it difficult for man to be commanded to be a source of blessing. How does one go about carrying out such a directive? Ed.] We encounter a similar construction as an imperative when G’d orders Moses to ascend the mountain and to die there on the mountain, עלה ומות בהר, seeing that “dying” is understood to be something passive, not active. Hence he would translate that verse also as “ascend the mountain where you will die.” (Deuteronomy, 32,48-50) Similar constructions which describe something impossible are found in Ovadiah 1,4 ואם בין כוכבים שים קנך, where one cannot order someone to make his nest among the stars. These formulations are substitutes for regular future tenses. G’d is telling Avram that his blessings and his fame will be so great that they will spill over to benefit those around him. This promise has become fulfilled to such an extent that when people bless their children they wish them: “may the Lord bless you with the blessing of Avraham”

ואברכך, “I shall bless you;” I personally, not through any agent of mine.

ואגדלה שמך, “I will make your name great.” I will add a letter to your name, so that its numerical value will amount to 248, corresponding to the number of limbs in your body. Your body will then be found to be whole, matching your name.