The basic difference between "Enhancement mode" & "Depletion mode" FETs:
Your text is probably based on a historical situation which is now much less common.
The very large majority of modern MOSFETs are "ehancement mode" devices.
They are "turned off" when the gate is grounded and need a minimum amount of foward bias - know as Vth or Vgsth (1) or similar - in order to conduct some specified small amount of current - often in the 10 uA to 100 uA range.
For N channel MOSFETS the gate bias is +ve relative to source to turn them on.
For P channel MOSFETS the gate bias is -ve relative to source to turn them on.
Note 1: Vth = Vthreshold, Vgsth = V gate-source threshold
A number of early FETS were depletion mode devices that needed reverse bias to pinch off the gate channel, and which conducted when the gate was grounded. While some of these are still available they are rare and seldom used in new designs.
Wikipedia - enhancement & depletion modes
EE Times - and introoduction to depletion mode FETS useful.
From the above page:
Wikipedia - Field effect transistor - some useful mention
Wikipedia MOSFET - some useful mention
Digikey depletion mode FETS - 99 items
Digikey enhancement mode FETs - 12465 items