What's the context, Justin? Usually it means how much of the available CPU time is being monopolized by a single process or program...
Your CPU (the brain of your computer) gets 'shared out' to various different tasks, because it doesn't need to be focused on any one thing... it sort of rotates through them, making it look like it's doing a bunch of things at once. Some tasks are simple... like, for instance, playing a music CD. Most of the work is done by the sound card.
Others are complex, like re-sizing or re-colouring a picture. Complex tasks use more CPU time.
Sometimes I get a little green box at the bottom right hand part of the screen by the clock.. It looks like a graft thing.. if that makes any sence.. Its not anything to be worried about, is it?