Feedback is a process in which information about the past or the present influences the same phenomenon in the present or future. This concept is used in sequential circuits.
Logic gate circuits are designed to input and output two types of signals only. That is high - 1 and low - 0
and represented by variable voltage.
1- Full power supply voltage
0 - Zero voltage
Even though specific voltages are assigned to these two in reality, logic signal voltage levels rarely attain these perfect limits due to stray voltage drops in the transistor circuitry.
Drawn above is an SR Flip Flop. Here Q(t),R,S all three are inputs Q(t) however is the feed back from the state before. In other words Q(t) is a feed back to the new state from the previous state.
A much simpler table would be;
When S=1 and R=1 the Q(t+1) value is undefined because at this state Q becomes equal to Q'.
SR flip flop is modified and JK Flip Flop is made to avoid this illegal status.