When a mechanical power switch is operated, the power is not available instantly.
The switch contacts bounce, giving a series of pulses, as shown in the graph.
The circuit may see these as a series of logic pulses and behave incorrectly.
The circuit shown prevents this.
If we revise the SET/RESET flip-flop, we will recall that once Q has been pulsed high by the SET input, further pulsing produces no change at Q.
Therefore Q only recognises the first pulse from the bouncing switch and ignores any others.
Similarly, when Q is taken low by taking RESET high, only the first pulse is recognised.