Motor controller diagram
The intention of the diagram is to show that the feedback system compares the intended and actual speed of the motor and derives an error signal which is used to alter the power supply in such a way that the motor speed changes towards what it should be. Thusly:



The diagram is confusing in that it seems to imply that the output of the error  comparator is amplified and used to directly control power supply voltage or perhaps frequency or ... in order to linearly control the motor based on the error amplitude. 

In fact, what happens is the amplified error signal is used to alter the power supply output in a manner which will reduce the subsequent error. eg if the motor is running faster than desired the sum of desired-actual will be negative and the power supply will be controlled in such a way as to reduce speed. I purposefully avoided direct statements about exactly how the error signal is used to affect speed. Because ...

Depending on the motor technology used and load characteristics the process of altering motor speed may be very simple or extremely complex. In a simple DC motor drive the system may reduce motor voltage. In some systems it may increase field excitation. In a process which involves complex dynamic effects a traditional PID control algorithm or a custom fuzzy logic algorithm (or both or more) may be applied.