A fatbike is a special kind of bicycle with really big tires. We're talking way bigger than the regular bicycle tires you're used to seeing! These tires are usually 4-5 inches wide, which is about twice as wide as normal bike tires.
Because the tires are so much wider, fatbikes are really good at riding on soft surfaces like sand, snow, or mud. The wide tires help distribute your weight better so that you don't sink into the ground as easily. It's kind of like having snowshoes or a raft for your bike!
Fatbikes can also be ridden on regular bike trails, but they might be a little slower and harder to pedal because of the bigger tires. But, they're really fun to ride and can go places where regular bikes can't! Plus, they look really cool!