What does Genesis 12:3 mean?

Verses 1–3 introduce the beginning of God's relationship with Abram; this is the earliest history of the nation of Israel. Without any recorded preamble, God shows up in Abram's life with a command to go away from where he is to an unnamed place. In the going, God promises to make Abram a great nation, to bless him, to give him a great name, and to make him a blessing.

Now God continues these promises: God will bless those who bless Abram. God will curse those who dishonor Abram. In Abram, God will bless all the families, or "peoples," of the earth.

God's words to Abram stop there. The next verse will reveal Abram's response. Notice, though, that the only condition for receiving these promises seems to be that Abram goes. Aside from that, God's promises here are not dependent on Abram's actions, or obedience, or goodness, or worthiness. God simply says He will do these things to and through Abram. Period. This parallels the fact that Abram's prior life and personality are given no description: his character is irrelevant to the purposes of God in this instance.

Knowing what we know now about the history of God's relationship with Israel, it's clear that God does indeed keep these promises (Deuteronomy 34:1–4; Joshua 1:1–9). Of course, Abram currently has no way of knowing that for sure. Instead, he will have to trust God to keep his vows (Hebrews 6:13–18).