An electric circuit is a path that electricity flows around. A simple circuit can have a battery (the energy source), wires, a bulb and a switch.
Electricity can only flow if the circuit is complete — a closed loop with no gaps. If there is a gap anywhere, the circuit is broken and the bulb will not light.
A switch is a way of making or breaking a gap. When the switch is closed (on), the loop is complete. When it is open (off), there is a gap.
Conductors let electricity flow through them easily. Metals such as copper, iron and steel are good conductors. Insulators do not let electricity flow through them easily. Plastic, rubber, wood and glass are insulators.