The commutator
If all the insulation round the wire of your motor's coil had been removed, the coil would rock back and forth instead of turning continuously one way. To prevent that, only one half of each end was stripped.
Then, as the coil turns, current flows only during half of each turn. Because current flows only half the time, the load the motor can turn is limited. So it is better to make the current flow one way in one half of the turn and the opposite way in the other half. The commutator is used to change the direction of the current like this.
Figure to be added
How it is made
The commutator is made of two metal split rings like those in figure 13.13, and two parts that rub on them called brushes. The two ends of the coil are joined to the two split rings, which turn with the armature. The two brushes do not turn: they stay in contact with the split rings (half rings) and are connected to the outside circuit that supplies current to the motor.
Why split the ring
Each brush always touches whichever half ring is on its side at that moment. As the coil turns through the upright position, the gaps in the ring pass under the brushes and each brush slides from one half ring onto the other. The brush joined to the positive terminal now feeds the other end of the coil, so the current in the coil reverses — exactly when the couple would otherwise start to push the coil back. The outside circuit never notices: the battery keeps pushing current the same way, and the swap happens inside the motor. This is what lets the motor use the current all the way round.
The commutator reverses the current each half turn
Split rings turn with the coil; fixed brushes press on them. Every half turn the rings swap brushes, reversing the current in the coil.