That's a simple circuit where a motor is connected to a battery. The switch is initially open so there is no current flowing trough the motor. Of course, once we close the switch, the potential will be accross the motor and the motor will start to spin.
A motor consists of a bunch of windings of cupper wires, so basically we can initially think of it as it was just a resistor when it is yet not spinning. Likewise, we can calculate the current due to the Ohm's law. It would simply be I = V/R , whereas V is the potential on the battery and R the total resistance of the motor.
However, once the motor starts spinning, due to the Lenz law, the total potential driving the motor is decreasing. The induced opposite voltage produces a counter-electromotive force (EMF or back EMF). You may also refer to the opposite voltage as a counter-electromotive force. In my opinion however, it is a little bit misleading.
So basically, when motor is spinning, then the curcuit experiences the EMF - the total driving potential is smaller, which contributes to a smaller current flowing accross the curcuit.
And that's really reallly good, because the heat generated by the motor is proportional to the current. So when the motor is left to run freely, there is less current running through the wires and the motor stays cooler.
When you put more load on the motor or it just starts, there is no EMF and the current is significantly larger. If we kept it this way, there would be some smoke and that would be the end of our motor.
Just for the sake of completness, I also give you the short definition by Wikipedia:
The counter-electromotive force (abbreviated counter EMF, or CEMF), also known as the back electromotive force, is the voltage, or electromotive force, that pushes against the current which induces it.