Commentary for Genesis 22:18

והתברכו בזרעך כל גויי הארץ, if and when your descendants will join together in proclaiming the holy name of the Lord, the nations of the world will be blessed as they will try to follow your example and to be like you.

והתברכו, the meaning of this word is not identical with the wordונברכו in 12,3 which we explained at the time as meaning that wherever you would reside the people around you would benefit from you or your children’s proximity. In this verse the meaning of the word והתברכו is that the gentile nations, thanks to their prayers, will accumulate merits and therefore blessings for themselves. They will make the kind of blessings Avraham has experienced the focus of their own prayers. They will acknowledge Avraham’s G’d as the source of all blessings.

And through your seed will be blessed, etc.: This is a blessing in of itself - that they should find grace and kindness in the eyes of the nations of the world to the point that they be blessed by them.