Leviticus 22:18

Speak with Aharon and with his sons and with all the children of Israel: -A man, whether young or old, of the house of the family of Israel, or of the strangers who are in Israel, who shall offer his oblation of any of their vows, or their free will offerings which they present before the Lord for a burnt sacrifice,

Scripture continues the general topic of the sacrificial offerings, now telling us that a blemished animal may not be sacrificed.

or a resident alien The laws concerning vows and pledges apply equally to Israel and to aliens, as it is written [Numbers 15:16]. Our Sages have distinguished between a “vow” and a “pledge” [Qinnim 1:1]. Every vow eventually becomes a pledge, but not every pledge ultimately becomes a vow. Any offering which is entirely burnt upon the Altar should be entirely perfect.

FOR ALL ‘NIDREIHEM’ (THEIR VOW-OFFERINGS) AND FOR ALL ‘NIDVOTHAM’ (THEIR FREEWILL-OFFERINGS). “Nidreihem (their vow-offerings). This is when one says, ‘I pledge myself [to bring an offering].’ Nidvotham (their freewill-offerings). This is when one says, ‘This shall be [an offering].’” This is Rashi’s language. It appears to me that because Scripture states ‘l’phalei’ (to clearly utter) contains also the letters of the word pele (wonder), and accordingly Ramban interprets the verse in this light. See Note 125. in fulfillment of a vow or for a freewill-offering, and similarly it always uses this [identical] expression in the section dealing with the drink-offerings, and likewise it is used in the verse, when a man ‘yaphli’ (shall clearly utter) a vow of persons, and so also in the section dealing with the Nazirite — [therefore it appears] that “the vow” is something which comes in connection with that which is beyond our power, the person in distress vowing unto G-d that “if You will deal ‘wondrously’ or yaphli, for since the root is pele (wonder, surpassing, or extraordinary) it suggests that the vow-offering is generally made when a person is in distress, and vows that if G-d will do wondrously with him he will bring an offering. with me to save me from this trouble, then I will bring a burnt-offering or peace-offering,” similar to that which is stated: And Jacob vowed a vow, saying: ‘If G-d will be with me;’ And Israel vowed a vow unto the Eternal, and said: ‘If Thou wilt deliver this people into my hand;’ and they vowed vows, and so also all similar verses.
And our Rabbis have said: “Which is a vow-offering? When he says, ‘I pledge myself to bring a burnt-offering.’ [Which is] a freewill-offering? When he says, ‘This [animal] shall be a burnt-offering.’ And what is the difference between a vow-offering and a freewill-offering? In the case of a vow-offering, one is responsible for it [and must replace it] if it dies or is stolen, [since he took upon himself a general obligation to offer an animal], but in the case of a freewill-offering, he is not responsible for it if it dies or is stolen” [since he did not assume a general pledge upon himself, but merely said that this particular animal should be an offering]. Now the reason for this difference between them [a vow and a freewill-offering], is because the custom of those vowing in the time of their distress is to say, “If G-d will be with me [and guard me] in this [trouble], then I pledge myself to offer a burnt-offering or a [peace-] offering,” and this is not usually called a freewill-offering, [since he has not yet set aside any particular animal for it]. But [a vow which is followed] by setting aside immediately [an animal] and giving it, is called “a freewill-offering,” because from the moment of giving, his spirit made him willing. Thus Scripture is stating here that whether he brings a burnt-offering or a peace-offering which he vowed at the time of his distress, or whether he brings them because his spirit made him willing — in either case it must be perfect [without blemish], for perhaps we might have thought that [it is more important that] the vow-offering should be perfect than the freewill-offering. This is the meaning of the verse.

אשר יקריב קרבנו, after the Torah had addressed the priests, who offer the sacrifices on the altar as well as their status of sanctity, it now discusses rules applying to the offerings themselves., saying that the following applies to לכל נדריהם ולכל נדבותם, even these offerings were voluntary offerings as opposed to mandatory offerings, and I might have thought that any such offering, even of a blemished animal, would be gratefully accepted by G’d, seeing the individual in question was under no obligation to offer any sacrifice at this time, the Torah states flatly that this is not so. The author, quoting Maleachi 1,5 as proof that one would not dare offer a gift to a human ruler that was less than perfect, how much less would one dare to do this “for” G’d, concludes that there must have been people in his time guilty of mistaking what is expected of them and offering inferior animals as sacrifices.