Aaand dumb question of the month goes to....

Boy have I got the stupidest question ever, LOL. Sometimes icon makers offer .psd files in their tutorials for download. What in the heck do you do with the .psd once you download it?

0.o x100

I've never experimented with .psd files before, hence with the cluelessness. :) Any help is, as always, much appreciated.

*slinks off*