When closing S1 current flows thru the 100nF capacitor into the base of T1. This triggered the monostable multivibrator into the active state. After x time DC current is blocked by the 100n capacitor. The monostable multivibrator stays on for t=In(2)*R*C=0.6973*100k*100uF=7s. After 7 seconds the monostable multivibrator goes to inactive mode, even if S1 is closed. If the monostable multivibrator is active and S1 is opened, the monostable multivibrator triggers off. This circuit is useful when x device cannot be on for more than x second even when the switch is closed.
