Imagine you have a toy box and you want to share it with all your friends. That's just like a computer that lots of people can use at the same time. This is what we call multi-user. It's when more than one person uses a single device, like a computer, and everyone gets their own space and things to play with.
Think of it like a clubhouse. Each club member has their own place to store their toys, and they can decorate it however they want. They can also play with their toys without it affecting the other club members' toys. In a computer that has multi-user capabilities, each person has their own account with their own files and programs, and they can use the computer without interrupting someone else's work.
So, just like how you share your toys with your friends in your clubhouse, multi-user computers allow different people to share one device without interfering with each other's work.