The 555 timer in bistable mode can be considered as being a flip-flop circuit. A flip-flop circuit can be imagined as flipping a light switch ON (flip) or OFF (flop). In other words, a flip-flop circuit can alternate between two stable states (ON/OFF, or 1/0).
In bistable mode, switching between states depends on a manual input. Monostable or astable modes on the other hand alternate between states automatically (making them a timer). Bistable 555 timer is a fairly basic timer circuit and does not require any timing. The state of the timer is controlled by the trigger pin and reset pin.