The speed at which bits (units of binary information) are transferred or processed over time. For audio data it can be thought of as analogous to resolution for image files. The greater the number of bits per second, the more detailed and clear the audio can be. In video it doesn’t affect the resolution in terms of the number of pixels in a frame, but it does affect how frequently and accurately they are updated. A low bit-rate usually manifests itself as blurry or blocky video with unwanted visual artefacts.
