Mosfet circuit

A MOSFET can act like a switch. When the gate voltage is low (below a certain threshold), the drain and source are not electrically connected, and so the transistor does not conduct (this is equivalent to the switch being open). When the gate voltage is high (above a certain threshold), there is a conductive pathway between drain and source (which is equivalent to the switch being closed). But it won't be a perfect conductor. There will be certain resistance between drain and source that appears when the transistor is "on" and conducting. This resistance is represented by R_ON. I shouldn't say more.