Ableton's Pedal device gives you three distortion characters to choose from, and this quick walkthrough breaks down what each one does. Overdrive delivers warm, smooth grit, Distortion pushes things more aggressive, and Fuzz is the extreme end of the spectrum for when you want a sound to fall apart in a good way.
Once you pick a mode, the Gain dial controls how hard you're driving into that distortion character, more gain means more saturation piled on. That extra intensity often comes with a volume jump, so the Output dial is there to bring the level back down without undoing the tone you just dialed in.
Thinking of these two controls as a pair, character first, then gain for intensity, then output for level, makes it easy to push a sound hard without it overpowering the mix.