In monostable multivibrator there is one stable state and one astable state. This multivibrator need a trigger (external signal) to enter into the astable state and get back to the stable state after some time period. The time period will be set by the user, the monostable multivibrator is mainly used as timer. Click on the link - how to operate Monostable Multivibrator
to get detailed information about this circuit.