The monostable has only one permanent stable state. When triggered by an external pulse, it changes over to an unstable state for a time determined by a CR time constant. It then reverts to its stable state and waits for another trigger pulse.
At switch on, Tr1 is forward biased by R3. This turns Tr1 hard on, giving it
a high collector current and a low collector voltage. This low collector voltage is cross connected to the base of Tr2, turning Tr2 off.
This is the stable state.
A negative pulse to the base of Tr1 turns Tr1 off. The collector voltage of Tr1 goes high and turns Tr2 on.
The circuit is now in the unstable state.
C1 now charges from the supply rail via R3. Eventually the voltage on the left hand side of C1 will be high enough to turn Tr1 back on, which in turn switches Tr2 off.
The circuit is now back in its stable state.
The monostable can be used as a short duration timer or a pulse width