Distortion is a post-effect. It amplifies the audio signal of a connected Source block to the point where the waveform starts clipping. This is where the peak amplitude exceeds the normal allowable range ( it’s “overdriven”), and the extremes of the peaks get cut off, or “clipped”.
This boosted and clipped signal creates a sound which is compressed with added overtones, making it more intense and in some cases can be described as warm, or dirty.
In the options, ‘Drive’ lets you set the amount the signal gets overdriven, and ‘Mix’ blends between this overdriven sound and the original audio.