After Tom and Becky get lost in the riverside cave at the picnic, terror ensues. Becky and Tom's guardians are frightened by their absence, and Becky and Tom themselves must face an uncertain end when they discover that the murderous Injun Joe is also hiding out somewhere in the cave's labyrinth.
Tom and Becky's last candle burns out, and they are forced to wander around in the darkness; thankfully, they manage to escape after five days, having traversed five miles from the primary entrance. An iron door is erected at the cave's entrance to prevent others from getting lost in the cave, effectively sealing Injun Joe to his death. His body is uncovered when Tom announces that the man had been in the cave with them.
Finally, in the novel's resolution, Tom and Huck sneak back into the cave and discover a treasure that had been hidden by Injun Joe: twelve thousand dollars in gold coins. With his new inheritance guaranteeing him a stable future, Huck is adopted by Window Douglas and agrees to behave under her roof, knowing that he and Tom will soon form a pirate gang.
The resolution is the solution to the conflict. At the end of the book, Tom and Becky are trapped in the cave. When they are rescued from the cave and find the treasue, and return home, that is the resolution of the story because the biggest confict is resolved.
It's a little hard to say that there's one resolution since this is such an episodic work -- it doesn't have one main plot. However, I would say that the best choice is to say that the resolution comes when Tom and Becky find their way out of the cave and Injun Joe dies. These two events resolve a couple of the major tensions in the novel (whether Tom and Becky will get out, whether Injun Joe will get Tom and/or Huck).