To expand on Sebi's (short) explanation:
A monostable multivibrator gives an output pulse of a fixed length, when it receives a trigger signal.
As you probably gathered, there are "retriggerable" and "non-retriggerable" types.
Retriggerable types will restart the timing of the output pulse, if it receives a second trigger, while the output pulse is still active. ie, if a certain multivibrator has an output pulse length of 10ms, and it is triggered, then receives a second trigger 5ms later, the output pulse will be (5+10)ms = 15ms long. Generally, they can retriggered as many times as you like.
Non-retriggerable types must finish the output pulse, before a new trigger signal will have any effect. (It will ignore any triggers whilst the output is active).