Here's an analogy for Defragmentation. Imagine that your computer is a line of coat hooks. Everytime you create a file, a coat is added to the line. When you delete a file, the coat is taken down, but not replaced. When you defragment, you take all of the coats down and put them back up with no spaces in between them. After that, you can add or remove even more coats. And then you eventually defragment again and fill up the spaces.