MOSFETs act like valves for electricity. They have one input connection (the "gate") which is used to control the flow of electricity between two other connections (the "source" and "drain"). Said another way, the gate acts as a switch that controls the two outputs. Think of a dimmable light switch: the knob itself selects 'ON', 'OFF', or somewhere in between, controlling the brightness of the light. Think of a MOSFET in place of the light switch: the switch itself is the "gate", the "source" is the power coming into the house, and the "drain" is the light bulb.