What does Genesis 15:7-18 mean?

And he said to him, “I am the LORD who brought you out from Ur of the Chaldeans to give you this land to possess.” But he said, “O Lord GOD, how am I to know that I shall possess it?” He said to him, “Bring me a heifer three years old, a female goat three years old, a ram three years old, a turtledove, and a young pigeon.” And he brought him all these, cut them in half, and laid each half over against the other. But he did not cut the birds in half. And when birds of prey came down on the carcasses, Abram drove them away. As the sun was going down, a deep sleep fell on Abram. And behold, dreadful and great darkness fell upon him. Then the LORD said to Abram, “Know for certain that your offspring will be sojourners in a land that is not theirs and will be servants there, and they will be afflicted for four hundred years. But I will bring judgment on the nation that they serve, and afterward they shall come out with great possessions. As for you, you shall go to your fathers in peace; you shall be buried in a good old age. And they shall come back here in the fourth generation, for the iniquity of the Amorites is not yet complete.” When the sun had gone down and it was dark, behold, a smoking fire pot and a flaming torch passed between these pieces. On that day the LORD made a covenant with Abram, saying, “To your offspring I give this land, from the river of Egypt to the great river, the river Euphrates,

English Standard Version (ESV)