Computer memory is like a desk. The bigger the desk, the more things you can out on it. When the desk gets full, you have to put something back in the filing cabinet before you can get out the new thing that you want to work on. This slows the computer down.
We can increase the size of the desk by fitting more RAM or we can reduce the number of things sitting there and taking up space. Lots of programs get themselves out ready even though you may not use them!