A bitcrusher is an audio effect that consists of two parts: sample rate reducer and bit depth reducer, which is sometimes known as a decimator. Depending on how extreme each of the two parts are applied and if/how they are modulated, the effects can range from creating lofi/retro sounds or extreme distortion and artifacts.
A side effect of sample rate reduction is [aliasing] which can be quite severe. It can be counteracted, and many bitcrushers include a built-in low-pass [filter] to handle this.