What does a motor driver do?
Refer to the image below:



Your microcontroller and motor driver take power from main power rails (there might be two different power rails for both). However the need for motor driver arises because your micro-controller is not powerful enough. It has GPIO pins which can help you glow a couple of LEDs but they won't provide you enough current to drive a big motor.

In such cases, you need a motor driver. They are specifically built for this purpose. They take the power from power lines and dump into the motor. However they lack the brains.

For this, we again turn back to the micro-controller, which tells the driver what to do. As such, micro-controller controls the motor indirectly.

Bonus analogy:

Lets say you want to beat up a bully at your school but you don't have the muscles. So, you ask for support from the friend who spends his mornings and evenings at the gym doing some heavy lifting. However you still got to tell him who needs to be beaten and how much. You are micro-controller and gym friend is your motor driver. The bully is the motor.