A bistable circuit is one that is stable in one of two states and will not change its state until the other switch is pressed.
The two inputs are made by two simple press switches, S and R.
If C = 1 and D = 0, LED 1 will be ON and LED 2 will be OFF. R and S are open and so E and F are high (1).
We call this the SET STATE. Pressing or releasing S will not alter the output.
R is now pressed. F becomes low (0) and so D becomes high. A now becomes 1 and so, as E is already 1, C goes low, LED 1 goes off and LED 2 comes on.
We call this the RESET STATE.
Pressing or releasing R will not alter the output).