The threshold voltage, commonly abbreviated as Vth or VGS (th), of a field-effect transistor(FET) is the minimum gate-to-source voltage differential that is needed to create a conducting path between the source and drain terminals.
For an nMOS device at gate-to-source voltages above the threshold voltage ((VGS > Vth) but still below saturation (less than "fully on", (VGS − Vth) > VDS), the transistor is in its 'linear region', also known as ohmic mode, where it behaves like a voltage-controlled variable resistor.
How It is affecting Timing
Low VT Cells :- these cells are faster but results in more power consumption.
High VT Cells :- these cells are slower than low vt cells, consumes less power than LVT cells
Based on requirement we need to use HVT and LVT cells since there is always trade off between power and speed.