In the circuit shown you recognize the additional input g. This input controls the multivibrator.
Only if you press the pushbutton switch S1, the LED flashes. Releasing the pushbutton switch and the LED goes dark.
In the illustration to the right, this is shown schematically.
This circuit is referred to as a "gated astable multivibrator". It is an important circuit in electronics, as again and again an astable multivibrator is required, which can be switched on and off electronically (quite analogous to the warning blinker from Experiment 10).
But now... to the most important component in the setting up of multivibrators...