The first adventure Bilbo finds once he, Gandalf, and the dwarves leave Bag-End is a run-in with these three trolls, Bert, Tom, and Bill. These guys are dangerous, sure – they enjoy eating "manflesh" (2.40) and they fight violently at the drop of a hat. At the same time, they're kind of funny: they're bumbling and incredibly stupid. In fact, their stupidity is what saves the dwarves from getting eaten.
The whole company is cold and hungry and sitting in a forest one night when they see a light through the trees. Bilbo goes to see what it is. There are three trolls sitting around a fire and roasting mutton (as in, sheep meat). Bilbo tries to prove his worth as a burglar by stealing one of the their wallets. But the wallet has been enchanted, and immediately says, "'Ere, 'oo are you?" (2.45). So Bilbo gets caught, of course. And in trying to rescue him, all of the dwarves get captured too and tied up in sacks.
They're in a pickle: Bilbo is standing at the side of this troll-filled glade, listening to the trolls argue over "whether they should roast [the dwarves] slowly, or mince them fine and boil them, or sit on them one by one and squash them into jelly" (2.75). Just then, a voice that sounds a lot like Bill's steps in to encourage their argument. Every time the trolls seem set to agree on something, someone who sounds like Bill or Tom or Bert will start the conflict all over again. Of course, this voice is Gandalf's, and he keeps the trolls arguing until the moment the sun comes up and they all turn to stone. Brilliant!
This episode in The Hobbit quickly illustrates what kind of a novel this is: there's going to be danger and the threat of death, but there is always also an edge of humor to the action. And Bert, Tom, and Bill also provide a good warm-up for Bilbo to get to know what the risks of his burglar job might involve before he actually has to face the real challenge of the goblins in the Misty Mountains. These trolls also bring Bilbo and the dwarves unexpected profit: they break into the trolls' cave and steal all of their gold and beer and bread. They also make off with three important weapons: Thorin's sword Orcrist, Gandalf's sword Glamdring, and Bilbo's long knife, which he calls Sting. These blades all come from the elves' wars with the goblins many ages ago, so they glow in the dark when goblins are near.