What is the gradient in photoshop and what can it do?

like this:a href="http://i247.photobucket.com/albums/gg159/caseybasey555/lostlve.png" rel="nofollow"http://i247.photobucket.com/albums/gg159…/a by photoshop. i like the way they do their backrounds and all. can you tell me please? i would love it if you did.