A DC motor is a motor that runs using a DC power supply.
As shown in the preceding picture, it has two terminal wires. We can call one of these terminal wire positive and the other one negative. Usually, these wires are marked red and black where red is positive and black is negative. Although these wires are marked positive and negative, unlike the battery, where these polarities can't be mixed, here in the DC motors they can be mixed.
Say, for example, on a motor, if the positive wire is connected to the positive terminal of the battery and the negative wire is connected to the negative terminal of the battery, or the power source, the motor rotates in a clockwise direction. When you reverse the connection, say from the positive terminal of the motor to the negative terminal of the battery and the negative terminal of the motor to the positive terminal of the battery, then the motor will rotate in an anti-clockwise direction, as shown in the following picture:
