To understand that first note that enhancement MOSFETs are normally OFF device. Now what is meant by normally off device?
Means if we apply a source to drain voltage then no current will flow.
The reason behind is that
There are 3 regions based o the application of gate to source voltage.
Suppose we are considering the n-channel enhancement type MOSFET.
For that n-channel enhancement type MOSFET there are 3 regions and they are accumulation, depletion and the inversion.
Now suppose for the n-channel enhancement type MOSFET we applied a fixed drain to source voltage and we are varying the gate to source voltage -2,-1,0,1,2,3,4,5.
Now what will happen when the gate to source voltage is-2 volt. Suppose source is at potential 0V and drain is at potential 5 volt.
Now when the source voltage is at zero volt and gate to source voltage is -2 volt then what happens inside the device. At gate the voltage is now-2 Volt. So at the one end of the insulator(silicon di-oxide) there is now -ve induced charges and at the other end(insulator –semiconductor interface at the insulator side).
Then as the induced charges at the insulator-semiconductor interface(at the insulator side) in negative now so at the semiconductor side positive holes will be attracted or accumulation region results.
This is called accumulation because its is actually the accumulation of holes for n-channel device(I mean n-channel MOSFTET).Then when we apply a positive voltage say 1 V at the gate terminal then due to positive charges induced at the semiconductor and gate and at the insulator and semiconductor interface positive charges induced and due to that fact there will be a depletion of the positive holes at the interface as the holes will now be repelled from the surface of insulator-semiconductor at the semiconductor site and the minority electrons will be attracted , for that when minority electrons are get at attracted towards that interface say still that number of electrons are not sufficient to from the channel. So still we will not get any drain to source current .This is depletion region(we call).Now when the gate to source voltage is greater than the threshold voltage of the device then the device will be on and at that time inversion will occur and a channel will be induced ,this is electrically induced channel not the channel present by structure as in depletion type MOSFET.
Now what is meant by inversion ?
The answer is that the p region in between the source an the drain will be inverted and will form a n-channel.
Say for example the device has the threshold voltage say 2 V.
So if gate to source voltage is greater than 2 volt then inversion will occur and we will get a current flowing from drain to source.