Could anyone explain to me what those 2 knobs on the bottom do exactly? I am only using the right line in jack, I can't figure out what the the right does. If I turn it to the left it becomes softer and to the right it becomes louder and distorted. I've just been putting it somewhere in the middle.

Input gain for the left and right inputs. Basically you set them by plugging in your guitar hitting it as hard as you possibly can and then adjust the knob to the point where the light stops coming on or only barely comes on.