Why does Addie call Jewel her salvation?
Addie calls Jewel her salvation because he is born out of love for another man (Whitfield). Since Anse has no real claim to the boy, Jewel, as a separate child from her loveless relationship with Anse, saves her from this empty relationship because of her love for him. Also, Jewel literally saves Addie's body from floating down the swollen river when the mules drown and the coffin is set adrift, so she may have had a premonition that he would somehow rescue her physically as well as spiritually.
In Section 39, entitled "Cora," Addie responds to the chiding of Cora that Jewel is her punishment for her sin,
"He is my cross and he will be my salvation. He will save me from the water and from the fire. Even though I have laid down my life, he will save me."
Literally, Jewel does, indeed, save Addie's decaying corpse from the river water and from the fire that Darl sets.
This literal salvation that takes place contradicts the implications intended by Addie who prophesied with these words; the term that is applied to such a circumstance is called ironic inversion. This episode is one of other such episodes in Faulkner's novel which is, at times, farcical in this tragicomedy.