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.