Why A monostable multivibrator, also called a one-shot, has exactly one stable state. A trigger pulse flips it into the unstable state, where it stays for a fixed period set by an RC time constant, then it falls back on its own. This makes it useful for generating a pulse of known width from an input edge of any length.
Watch out The continuous square wave describes the astable multivibrator, which has no stable state and free-runs as an oscillator, and storing one bit describes the bistable multivibrator, better known as a flip-flop.
Count the stable states: a-stable = none (oscillates), mono = one (one-shot pulse), bi = two (stores a bit).