A Variable-Frequency Drive(VFD) or Variable-Speed Drive(VSD) can go by a number of given names. The VFD varies the frequency going to the motor. The purpose of the VFD is to fine tune the rotations of the motor so that it may closely match the application demand. A motor running at 63% load will only consume 25% of the power that it would at 100% load.
A standard motor will have a simple on/off operation and the demand will be matched by turning the motor on and off. These harsh starts and stops shorten the life of the motor causing parts to wear faster. There is also a decrease in efficiency due to the start-up stress of the motor.
A VFD will gradually ramp up to the specified demand. This slow start means less wear on the parts and increased efficiency. If the demand changes the VFD can be tied to the BAS and programmed to speed up/down accordingly. It is much more efficient to have the motor running for hours instead of started and stopped every few minutes.