Q.What is I_DSS?

The working of an n-channel JFET is understood by watching how the gate voltage controls the drain current.

With both supplies connected, the drain supply drives electrons from source to drain through the n-channel, forming the drain current $I_D$, while the gate supply reverse biases the gate junctions. When the gate-to-source voltage is zero (gate open), the channel is at its widest and the drain current reaches its maximum value, called the drain-source saturation current $I_{DSS}$.

As the reverse gate voltage is increased, the depletion regions push further into the channel, the channel narrows, and the drain current falls. Increasing the reverse bias still more reduces $I_D$ further, until at the pinch-off voltage $V_P$ the depletion regions meet, the channel is fully blocked, and $I_D$ becomes zero. So the device current is controlled mainly by the reverse voltage applied between gate and source. …