What does it mean in the KJV Bible when we read Genesis 15:17 "a smoking furnace and a burning lamp"?

In Genesis 15:13 God told Abram that his seed (Israel)will go into captivity for four hundred years (affliction), then in Genesis 15:14 Abram is told that his seed Israel) will be freed (deliverence).

According to E. W. Bullinger (The Companion Bible) (bottom) the smoking furnace is Symbolic for affliction and the burning lamp is symbolic of deliverence.

Genesis 15:13-17
13 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;

14 And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance.

15 And thou shalt go to thy fathers in peace; thou shalt be buried in a good old age.

16 But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full.

17 And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces.

furnace. Symbolic of the affliction of Israel (Deut. 4:20; 1 Kings 8:51; Isa. 31:9; Ezek. 22:18-22).

lamp. Symbolic of Israel's deliverance (1 Kings 11:36; 15:4. Isaiah 62:1. 2 Samuel 21:17).

From the Companion Bible