The diagram illustrates what is called a proportional controller, one of the simplest closed- loop control schemes.
The set point signal is subtracted from the tachometer signal to give an error signal, which is amplified and fed to the motor. For example, suppose the set point signal is 0-10V.. Imagine it at 50% of top speed .. if the motor is not turning, the tach signal will be 0V so the error will be 5V. Now a gain factor is applied .. perhaps something like 0.1V = full power. So, the motor driver saturates at full power until the speed approaches the desired set point. It will settle out with a maximum error of 1% (depending on load and set point) plenty good enough for many applications, and if the gain is not set too high(!) it will be stable. There is a trade off between stability and maximum error since real closed- loop systems will be at least 2nd order.
In the above example, if the motor requires 100% power to turn at 50% of top speed (say it is heavily loaded) the error will be 0.1V or 1% of full speed (4.9V from the tachometer) so if top speed is 10,000 RPM, it will actually turn at 4,900 RPM.