Electricity flows from one end of the battery, through the wires, bulbs and whatever else is connected, and back to the other end of the battery.
If one part of the circuit is not joined to the next, no electricity flows and the circuit doesn't work.
This is not always a bad thing. For example, we may not want to leave the bulb on all the time. A switch is a device that allows us to break and remake the circuit conveniently.
