It's labeled incorrectly - on most amps, it's the "Awesome" button. Seriously, though, overdrive is a type of distortion. The button switches the input to a hotter preamp that is specifically designed to distort the guitar. Examples of this can be heard on pretty much any guitar music that can be labeled as rock or blues in the past 60 years. Don't be afraid, hit the button and let the amp breathe. The higher the gain, the more you'll rock.