A mon is like a special symbol that tells people who you are. It's kind of like a badge or a flag that you can put on your clothes or things that belong to you.
Long, long time ago in Japan, people didn't have last names like we do now, so they used mon to show which family they came from. Each family had their own unique mon that they would use on everything from their clothes to their flags.
The mon usually had a picture of something that was important to the family, like an animal or a plant. Sometimes the pictures would be really simple, like a circle or a square. Other times, they would be more complicated and have lots of different colors.
Nowadays, mon are still used in Japan, but not as much as they used to be. Some families still use them, but they are also used by companies, sports teams, and even cities to show off their identity.
So, think of a mon as your special symbol that tells everyone who you are and where you come from. Just like a badge or a flag, it's an important way to show off who you are and what you stand for.