What is the meaning of Genesis 15:9-17?

God promises an heir and a land to Abram.

Gen 15:13 KJV
And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land [that is] not theirs, and shall serve them; and they shall afflict them four hundred years;

scripture cross-references to "Gen 15:13", where it says, "thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them" are below:

Act 7:6 KJV
And God spake on this wise, That his seed should sojourn in a strange land; and that they should bring them into bondage, and entreat [them] evil four hundred years.

Act 7:7 KJV
And the nation to whom they shall be in bondage will I judge, said God: and after that shall they come forth, and serve me in this place.

scripture cross-references to "Gen 15:13", where it says, "four hundred years;" are below:

Exo 12:40 KJV
Now the sojourning of the children of Israel, who dwelt in Egypt, [was] four hundred and thirty years.

Exo 12:41 KJV
And it came to pass at the end of the four hundred and thirty years, even the selfsame day it came to pass, that all the hosts of the LORD went out from the land of Egypt.

Gal 3:17 KJV
And this I say, [that] the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.


References:

https://www.blueletterbible.org
Lexicon/Strong’s Concordance
https://www.quora.com
https://www.quora.com/What-is-the-meaning-of-Genesis-15-9-17