An electric current is a flow of charge, and in a wire this will be a flow of electrons. We need two things for an electric current to flow:
The simplest complete circuit is a piece of wire from one end of a battery to the other. An electric current can flow in the wire from one end of the battery to the other, but nothing useful happens. The wire just gets very hot and the battery loses stored internal energy – it ‘goes flat’ and stops working.
To do something useful with the electric current, you need to put an electrical component into the circuit (such as a lamp), that can use the current in a useful way.
You usually add a switch to the circuit. This lets you break the circuit and stop the electric current when you want to.