You can think of the operation of any transistor as an ordinary faucet. The water enters the faucet in the pipeline from the water distributor, which would correspond to the source in the MOSFET. The water then leaves the faucet into the sink, this would be the drain in the MOSFET. The water tap controls the amount, flow, of water. In the MOSFET the gate operates as this controller. With a small force you can control the water flow with the water tap, just as you can control the current flowing from the source to the drain, with a small change of the charge of the gate.