Think of your hard disc as a bookcase and a file as a set of volumes of an encylopedia. When you first put volumes of books on an empty shelf, you put them in order. But as the bookcase fills up, you just put them where there is a space – perhaps where you have thrown a book away.
So the bookcase ends up a bit ragged and it takes longer to find the volume you want.
When you defrag the disc, it is as if you take all the books off and put them back on the bookcase in order, and that makes everything quicker