Variable frequency drives (VFD) are also known as variable speed drives (VSD) or adjustable speed drives (ASD). A VFD is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. By increasing the frequency, the motor can speed itself up and down as required by the control process.
When an electrical motor is started by a switch or a PLC, the motor draws a very high current. Until the motor is running at full speed, the current draw is very high. When a VFD is used, this amperage inrush can be mitigated by varying the frequency to the motor. The VFD can then gradually ramp up the frequency lowering the initial current demand.
Note that a VFD should not be confused with a soft starter (though it can be used in its place).