First off, a Mosfet is an electronic amplifying device, more specifically, a transistor. It is a little part that has three connecting pins coming out of it, known descriptively as the Source, the Drain, and the Gate. Using a hydraulic metaphor, if you think of the Mosfet as a faucet, then the Source and Drain are what the water flow through, and the Gate is the lever that turns the flow on or off, fast or slow. Except, of course, that the water is electrons, and the lever is operated by electronic charge.