I'll quote wikipedia here:

    The demoscene is an international computer art subculture that specializes
    in producing demos: small, self-contained computer programs that produce
    audio-visual presentations. The main goal of a demo is to show off
    programming, artistic, and musical skills.

Basically, these programs are extremely small, but produce interesting and impressive visual or audio experiences. A popular example would be "fr-041: debris. by farbrausch": https://www.youtube.com/watch?v=mxfmxi-boyo