In Genesis 15, Abram entered into a covenant relationship with God.
After he rescued Lot and encountered Melchizedek, the word of the Lord came to Abram in a vision.
The Lord (to Abram): Fear not, Abram: I am your shield, and your exceeding great reward.
Abram: Lord God, what will you give me, seeing I go childless, and the steward of my house is this Eliezer of Damascus? You have given me no children to me, and my heir is one born in my house.
The word of the Lord came to him.
The Lord: This man shall not be your heir; but he that shall come forth out of your own bowels shall be your heir.
In Genesis 12:2-3, God had told Abram that he would make him a great nation.
And I will make of you a great nation, and I will bless you, and make your name great; and you shall be a blessing: And I will bless them that bless you, and curse him that curses you: and in you shall all families of the earth be blessed.
Now God tells Abram specifically that he will have a son.
The Lord brought Abram outside.
The Lord: Look toward heaven and count the stars, if you be able to number them. So shall your seed be.
Abram believed in the Lord; and the Lord counted it to him for righteousness.
The Lord: I am the Lord that brought you out of Ur of the Chaldeans, to give you this land to inherit it.
Abram: Lord God, how shall I know that I shall inherit it?
Abram asked a question. God was not offended by the question. His answer was to make a covenant with Abram.
The Lord: Take a three years old heifer, a three years old she goat, a three years old ram, a turtledove, and a young pigeon.
Abram took all these and divided them in the middle, and laid each piece one against another: but the birds he did not divided.
When the fowls came down upon the carcasses, Abram drove them away.
When the sun was going down, a deep sleep fell upon Abram; and, a horror of great darkness fell upon him.
The Lord: Know of a surety that your seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years. I will judge that nation, whom they shall serve and afterward shall they come out with great substance. And you shall go to your fathers in peace. You shall be buried in a good old age. But in the fourth generation they shall come here again: for the iniquity of the Amorites is not yet full.
When the sun went down, and it was dark, a smoking furnace and a burning lamp passed between the carcass pieces.
Simplified, the symbolism here is God walking with Abram down a path of carcasses. It makes me think of Psalm 23:4.
Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me.
In that same day the Lord made a covenant with Abram.
The Lord: I have given this land to your seed, from the river of Egypt to the great river, the river Euphrates: the land of Kenites, the Kenizzites, the Kadmonites, the Hittites, the Perizzites, and Rephaims, the Amorites, the Canaanites, the Girgashites, and the Jebusites.