A potentiometer is what translates your stick movement into a digital signal. Its really nothing more than a variable resistor. The stick moves the pot, the pot changes resistance, and a circuit translates that in to where the stick is positioned and encodes it for transmission.

Expo deals with how your servo moves in relation to stick movement. Linear throw if you move the stick half way the servo moves half way.

If you enable expo the servo may only move 25 percent of its throw between center and half stick, then the other 75 percent from half to full throw. The effect is that the center feels softer but you still have full throw if you yank the stick around. Dual rates perform a similar function in that they make the center feel softer and the model more controllable but it sacrafices total throw.

If you want to play with it try 25 percent or so for starters, positive for softer center for JR and negative for Futaba.

You can also achieve the opposite effect making the center very sensitive if you so choose.

As for the lockout stuff, thats rubbish.