in order to understand the working of MOSFET. we apply voltage VPS drain and source and voltage vas between gate and source, such that gate is positive w.r.t source, due to positive voltage at gate positive charges are accumulated on the gate, which generates electrific field. this electric field penetrates into P substrate thro' $Sio_2$ layer and repels holes, when Vas = Vin where Vtn is known as threshold voltage then the electric field attracts the electrons present in the substrate, thus near the Si - sies interface we get lots of electrons i.e. a channel is created. these electrons present in the channel are attracted by battery vas thus. there is circulates and because of which we get the drain current ID.

Thus VTN is threshold voltage, defined as gate voltage at which MOSFET conducts and drain current in flows.