Why Do We Call Them " Flip-Flops" ?

Because they flip between two stable states, in the manner of a seesaw that comes to rest on one end or the other. So let’s use this analogy to help explain the operation of an RS flip-flop.

Consider the seesaw shown in Figure 2. It’s a bit rusty, and will remain at rest on either end when there’s no one on it. Now note the following:

◊ The left end of the seesaw represents output Q; the right end, output Q#.
◊ The people—R and S—represent the inputs. The input goes to logical H when a person gets on the seesaw, and to L when a person gets off. (The analogy is not perfect: We do not allow R and S to be on the seesaw at the same time!)

Suppose that R got on instead of S. In that case Q would be L (Q# would be H); and this state would remain after R gets off. We can see how the movement of the seesaw " remembers" which person was previously on board.
The truth table in Figure 5 shows how the RS flip-flop operates. In the table, Q0 and Q0# are the outputs in effect before the input change.

The RS flip-flop is the simplest of the four flip-flop types. It’s often used, for example, to prevent malfunctions of mechanical switches.