FET stands for "field effect transistor". Transistors are essentially switches. FETs are the switching mechanism through which the current flows from the battery to the motor. They are the basic mechanism that enables an ESC do to what does. Modern day FETs use materials like silicon carbide (SiC), gallium arsenide (GaAs), gallium nitride (GaN), and indium gallium arsenide (InGaAs). These are the materials that enable the transistors to do what they do.

Like anything else, transistors can be had in a wide variety of quality and performance. Because a speed controller must pass a large amount of current (ie: many electrons flowing at once), the FETs need to be able to facilitate that flow and not be a bottleneck or a source of excessive electrical resistance which creates heat.