Page "Data compression" Paragraph 59

Video compression typically operates on square-shaped groups of neighboring pixels, often called macroblocks.

These pixel groups or blocks of pixels are compared from one frame to the next and the video compression codec sends only the differences within those blocks.

In areas of video with more motion, the compression must encode more data to keep up with the larger number of pixels that are changing.

Commonly during explosions, flames, flocks of animals, and in some panning shots, the high-frequency detail leads to quality decreases or to increases in the variable bitrate.