Genesis 15:11 Meaning

9And he said unto him, Take me an heifer of three years old, and a she goat of three years old, and a ram of three years old, and a turtledove, and a young pigeon.

10And he took unto him all these, and divided them in the midst, and laid each piece one against another: but the birds divided he not.

11And when the fowls came down upon the carcases, Abram drove them away.

12And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him.

13And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not their's, and shall serve them; and they shall afflict them four hundred years;