Enhancement type nmos threshold voltage
Your Channel is Itself in depletion region

To understand whats happening imagine a freshly created MOSFET a just newly put together wafer(though they are diffused with impurity but just for example lets imageine),so as you are using NMOS your substrate is P type,and Source and Drain is N type.At first instant both the regions of the Source and Drain they have both carriers(e in S and D and holes in P substrate)  which will diffuse across the barrier creating a depletion region i.e there are no charge ,only immobile ions there,the diffusion will stop when  a voltage cause by the immobile ions exceed the gradient voltage of carriers.

This voltage now stops the diffusion of the carriers,and is called the Threshold Voltage.(Vth)

Now as your NMOS is enhancement type you will have no pre-made channel,all you have is depletion region lying between the DRAIN and the SOURCE.A channel is one which has free carrier to casue flow of charge,you already have a depletion region all you need is charge.
Voltage through gate that will be act against the Threshold voltage,and hence will create a voltage difference along with action of attracting electrons form the substrate to act as carrier in depletion region.Now By applying a suitable Drain voltage you can have a current flow through the channel.