Okay little one, have you ever seen a wagon? It's like a small car with no engine or pedals that you can pull around on wheels. You can put things in the wagon or even sit in it, and it's a fun way to move things around instead of carrying them yourself.
The wagon has four wheels that are round and roll along the ground. The front wheels steer the wagon so that it goes where you want it to. The wheels are attached to a frame which holds up the wagon and keeps everything together. Sometimes, wagons can also have a cover or canopy to protect the things inside from the sun or rain.
Long time ago, people used wagons to carry things like food, clothes and tools from place to place. There were no cars or trucks back then, so wagons were very important. Nowadays, wagons are often used for hauling kids, toys or groceries. They are still a fun way to move things around without putting in too much effort.
So, that's what a wagon is, a small vehicle with wheels that you can pull around to transport different things. I hope that helps you understand what a wagon is!