Multiseat configuration is like having multiple chairs in front of a computer, so more than one person can use the computer at the same time!
Normally, when you use a computer, you sit in front of it and use the keyboard and mouse to do things. But with multiseat configuration, you can add extra keyboards, mice, and monitors to the computer, so that more than one person can work on it at the same time.
It's like having a big table with lots of chairs around it, and each person has their own set of tools to work with. This is useful in places where multiple people need to use the same computer at the same time, like in a classroom or a library.
So even if you have just one computer, with multiseat configuration, it can be used by multiple people at the same time, each person working on their own "seat" and with their own set of tools.