Distortion is a form of audio signal processing used to alter the sound of bass tracks by increasing the bass signal’s gain through certain distortion devices. Distortion adds sustain, harmonic and inharmonic overtones, leading to warm and often 'gritty' compressed sounds depending on the type and intensity of distortion used.
The effects modifies the bass track's sound by clipping the signal, altering the original shape and sonic characteristic of an audio waveform. While unintentional distortion is often not wanted, added distortion can be a desirable audio effect and production aesthetic for many projects with very limited creative boundaries.
Distortion can be added in different ways such as driving an amplifier, using an effects pedal or by overdriving the input of a console channel strip. Different devices will produce a different character of distortion.