Okay kiddo, a mountain bike is a special type of bike that's designed to ride on rough and bumpy roads, rocky paths, and mountains. It has thick, knobby tires that help you grip the ground and go over obstacles like rocks and roots without falling.
The frame of the mountain bike is usually made of sturdy metal or carbon fiber, which makes it strong and lightweight. It also has a lot of gears, so you can change how hard it is to pedal depending on the slope of the trail or road you're on.
There are also different types of mountain bikes, like full suspension bikes with shock absorbers that absorb the bumps, and hardtail bikes that don't have any shocks on the back.
Mountain biking can be a fun way to explore the outdoors and get some exercise, but it's important to always wear a helmet and other protective gear, and to follow the rules of the trail. And remember, always ask an adult to help you when riding a mountain bike!