I often rename documents, folders and other items on my computer. For instance, if you download a bunch of photos from your camera, you don't want them named "11053.jpg" and such, do you? No, you want to give them meaningful names, like "DadFallingOffPorch".
You can always right-click on a filename in Windows and left-click "Rename" from the menu that pops up. Or you can use a shortcut and just hit the F2 key. That saves you mousing and clicking (is "mousing" a word? Well, it is now!) After pressing F2, the current filename will be highlighted. Just type the new filename and "Enter", and you're all set.