The 4066 contains 4 analogue bilateral switches, each with an active-high enable input (A) and two input/outputs (X and Y). When the enable input is set high, the X and Y terminals are connected by a low impedance; this is the on condition. When the enable is low, there is a high impedance path between X and Y, and the switch is off.
Analog switch integrated chips (ICs), when turned on, will conduct both analog and digital signals from the input pin to the output pin. An analog switch acts like a solid-state relay (i.e., with no moving parts).