In the chapter "I Spare Miss Watson's Jim," Huck meets Jim on the island where both have taken refuge after running away.
When the chapter opens, Huck is alone on the island and he grows confident in his safety there. When he comes upon a smoldering fire, Huck realizes someone else has come to the island. By the end of the chapter, Huck has discovered that this other person is Jim and has sided with Jim (morally) by declaring that he will not "tell on Jim" or turn him in to the authorities.
Huck spares Jim in two ways. First, when he finds Jim initially, Huck is carrying a gun. He does not shoot Jim and so spares him in this way. Also, Huck's agreement to stay silent about Jim's status as a run-away slave spares Jim by helping him maintain his freedom.
In siding with Jim, Huck realizes the gravity of his choice.
"People will call me a low-down abolitionist and despise me for keeping mum - but that don't make no difference. I ain't going to tell [...]."
Although Huck had been free from any obligations after running away from his father and faking his death, at this point in the story he opts to take on the responsibility of keeping Jim's secret. This leads to many more choices and actions on his part, despite the fact that his initial agreement was simply to keep quiet.
"As he protects Jim, Huck feels certain that he will go to hell. Nonetheless, he transcends his upbringing and learns to value essential human bonds of trust beyond his own interest" (eNotes).
In "sparing Miss Watson's Jim," Huckleberry Finn chooses a path toward independence of thought and moral growth. There were many other choices Huck could have made (leaving Jim alone to make it on his own or turning him in to the authorities). Yet he chooses friendship and that is, perhaps, the most consistent and honorable trait Huck Finn displays in both Tom Sawyer and Huckleberry Finn.
In this chapter, Huck finds Jim on the island with him. Jim admits he has run away from Miss Watson because he overheard her say she was considering selling him to slave traders. Huck agrees not to turn Jim in, despite the fact that people will think less of him for this.