The Levels command, and its cohort, the Histogram, let you adjust luminance levels on a channel-by-channel basis. The upshot is that you can increase contrast, correct for color cast, and make a bad image good.
Are you new to Photoshop? Have you been trying to teach yourself the basics of Photoshop but have found the amount of amount of educational material available on the net a bit overwhelming?
Here's a little video tutorial that will help you clear some things out!