Imagine you have a toy that you and your friends can all play with together. You all want to use the toy at the same time, but you can't all play with it at once. So, to share the toy, you decide to take turns. One person gets to play with the toy for a little while, then they give it to the next person, and so on.
Now think of a computer as a toy that many people want to use at the same time. This is called a multiuser system. Just like with the toy, people can't all use the computer at the same time. But instead of taking turns manually, the computer has a special program that helps it keep track of who is using it and when.
The program also helps the computer make sure that each person's information stays private. For example, if you have a secret diary on the computer, you wouldn't want your friend to be able to read it just because they're using the computer after you. The multiuser system makes sure that each person can only see their own information.
Overall, a multiuser system is like a really good toy-sharing plan for a computer. It helps lots of people use the computer at the same time, but also keeps everyone's information safe and private.