There are four controls:

Fuzz knob: This controls, as you might imagine, exactly how much fuzz is applied to the signal.
Tone knob: This will control the brightness of the distorted signal.
Output knob: This is the output volume of the device, when it is active.
Bypass button: This works just like the switch on a stomp box. When the orange light is lit, you're money. When it's not, the input is passed directly to the output. This can be automated for a slightly different vibe than automating the effect bypass of your host.