A hyperbole is another word for any intentional exaggeration for effect. If you think about it, we all use hyperboles every day in our speech. Consider the following example: "I've been walking for miles!" Mostly we haven't been walking for miles, but the hyperbole here conveys the sense of exhaustion we feel and our sense of how long we have walked.
Twain is a writer that uses hyperbole a lot in his work. Huck Finn, as a character who likes to embellish and exaggerate, uses hyperbole in lots of instances. Consider this example from Chapter Sixteen which describes the steamboat that smashes into the raft:
She was a big one, and she was coming in a hurry, too, looking like a black cloud with rows of glowworms around it; but all of a sudden she bulged out, big and scary, with a long row of wide-open furnace doors shining like red-hot teeth, and her monstrous bows and guards hanging right over us.
Note how the description is exaggerated to make the steamboat appear more fearsome and dangerous than it actually is. The similes and metaphors employed help in this hyperbole, comparing the steamboat to a "black cloud" surrounded by "glowworms" to increase the fear...
that the steamboat instills. Likewise the steamboat increases in size, with the furnace doors looking like "red-hot teeth," which exaggerates the size and appearance of the steamboat as it crashes into them.
This is just one example. Hopefully you will now be able to go back and find some more examples of hyperbole in this excellent novel. Good luck!