When you apply the clock signal to the E input of a flip-flop, you surely have noticed one thing. The time the signal is 1, is relatively long. It can indeed happen, that the open flip-flop changes state several times within one clock cycle.
It would be advantageous if the flip-flop changes its state only at an edge, when the clock signal switches from 0 to 1 or vice versa.
Below a master-slave circuits is shown. It results in an edge-triggered flip-flop with the inputs D and Clk (for CLOCK). This new flip-flop changes state only at a rising edge.
An explanation of how exactly a master-slave circuit works, goes too far at this point. However, you can also build other master-slave circuits resulting in an edge-triggered flip-flop switching on falling edges.
Now it is time to experiment with edge-triggered flip-flops.