What is the function of a commutator in an electric motor?

Hint: Commutator is a rotary electrical switch that periodically reverses the current direction between motor and external circuit.

Complete solution:
What is an electric motor?
An electric motor is a device which converts electrical energy to mechanical energy. The motor takes in an electrical energy as input and gives mechanical work as output. The electric motor works on the principle of current carrying elements in an external magnetic field. We all know that when a wire carrying a current is placed in an external magnetic field then the wire experiences a force.
The electric motor works on the same principle. The motor has a coil free to move between two magnetic poles. When the current flows through the coil, the two elements of the coil which are perpendicular to the magnetic field experience a force in the perpendicular direction which causes the coil to rotate.
The different parts of the electric motor are:
1. Armature.
2. Shaft.
3. Magnetic poles.
4. Commutator.
5. Brushes.
The function of the commutator is to reverse the current direction in the rotating winding each half turn which produces steady force. Reversing the direction with each half turn as mechanical rectifies to convert alternating current from the winding to unidirectional direct current in the external circuit.

Note: Commutators are needed on all DC generators and DC motors and also in AC motors such as reputing, synchronous and universal motors. The commutators act as rectifiers i.e. they convert the AC current into DC current and so cannot be used in an AC generator as we want the output as AC and not DC.