Dream tetris is a game you play when you are consciously or unconsciously imagining falling tetrominoes before your eyes. It is a game that requires no computer, or anything for that matter besides a healthy dose of imagination. Most people play dream tetris just before going to sleep or when zoning out in a boring situation. An interesting aspect of dream tetris is people rarely seem to lose, because all the blocks fall just the way they want them to.
It is common to play dream tetris with a small field, because that keeps things easy to follow. Experienced players might play in an actual 20-by-10 field. mf plays with a field of either 8 or 10 blocks wide, but infinitely high, so he can never top out. Rosti_LFC reports that he plays with a 4-block wide well, somewhat similar to BIG mode.
See also: The Tetris Effect