filesystem

A filesystem is a set of rules as to how files should be organised and accessed on a partition - this usually deletes everything on that partition (if there’s anything on there to begin with). Think of the partition as a piece of paper. Formatting is like drawing the guide lines, margin and setting the page layout so you can write inside of it properly, of course, for the piece of paper to be useful, everything on it needs to be rubbed out before you can do so.