Like many folk stories, the story of the "Three Billy Goats Gruff" can vary in different re-tellings and often finds its way into modern art, TV shows, movies, and video games. There is even a phone app game. You can find the full written story and cartoon videos on the internet.
Briefly, the story is about three goats with the same family name of "Gruff" who have run out of grass to eat where they live, so they must cross a river to get to the lush grass on the other side of a stream. To do so, they must first cross a bridge, under which lives a fearsome troll who eats anyone who passes that way.
The smallest billy goat is the first to cross and is stopped abruptly, by the troll who threatens to "gobble him up!" The little goat convinces him to wait for another slightly bigger billy goat to come across because he is larger and a more gratifying feast. The greedy troll agrees and lets the smallest of the goats cross.
The medium-sized goat then passes next. He is more cautious, but is also stopped by the troll and given the same threat. The second billy goat is allowed to cross as well after he tells the troll to wait for the third billy goat because he is the biggest.
The third billy goat then gets on the bridge and is stopped by the hungry troll. When the troll gets up on the bridge, however, the third billy goat is so big that he easily kicks the troll off of the bridge. The troll falls into the stream. From then on the bridge is safe, and all three goats are able to go to the rich fields around the summer farm in the hills. They all live happily ever after. The troll still lives under the bridge, but has not bothered anyone again.