Exodus 4:14

And the anger of the Lord was kindled against Mosheh, and He said, Is it not manifest before Me that Aharon thy brother speaking can speak? And, behold, also, he cometh forth to meet thee, and will see thee and rejoice in his heart.

I KNOW THAT HE [Aaron] CAN SPEAK WELL. That is to say, “It is revealed before Me that, out of his love for you, Aaron will willingly speak on your behalf even if I were not to command him. And also, behold, he cometh forth of his own bidding to meet thee, and when he seeth thee he will be glad in his heart, and he will not be jealous over your distinction in this honorable mission.” The reason that it was necessary that G-d tell Aaron, Go into the wilderness to meet Moses, was in order to inform him of the road by which Moses was coming. It is possible that Aaron heard of Moses’ departure from Midian, and on his own accord he went out to meet him. Afterwards, when he was already on the way, it was said to him, “Go into the wilderness to meet him, for there you will find him.”

ויחר אף ה' במשה, such an expression always means that G’d’s anger had tangible consequences for the one at whom G’d was angry. In this instance, the encounter described in verse 24 where G’d is described as seeking to kill him is such a consequence. I already explained on Genesis 32,29 that Yaakov’s having had to limp was a punishment for him.

כי דבר ידבר הוא. He grew to maturity in Egypt, and he has complete mastery of the kind of Egyptian spoken by the upper classes, so that he will be listened to with respect.

הלא אהרן אחיך? If it had been My intention to appoint someone naturally suited for the task, there is your brother Aaron, the Levite, undoubtedly a wise man as are all his brothers the Levites.

כי ידעתיו כי דבר ידבר הוא, for I am well aware that he is an excellent speaker without needing training, וגם הנה הוא יוצא לקראתך וראך ושמח בלבו, and although he is on the way to meet you out of respect for you and your elevation to an even higher status than his own, he will undoubtedly be happy to serve as your interpreter full-heartedly.

וראך ושמח בלבו, “as son as he sees you he will rejoice in his heart.” The reason why G–d had to tell him this was that Moses had not wanted to usurp the status of his older brother as he feared that this might lead to his becoming jealous of him. (Sh’mot Rabbah 2,27. G–d told him that he had no reason to worry about this, and that He had already commanded Aaron to be on his way to be reunited with his brother whom he had not seen or heard from for many decades. (Exodus 4,27) Moreover, G–d also told him that all the people in Egypt who at one time had sought to have him executed had already died so that he did not have anything to fear for the safety of his person.