A potentiometer is a variable resistor. A potentiometer has 3 leads, a right, middle, and left lead. As you turn the potentiometer to the right the resistance between the middle and the right lead decreases and the resistance between the middle and left lead increases, the opposite happens when you turn it the other way. The actual way to turn them and leads vary from potentiometer to potentiometer but thats the general idea. They come in different values rated in Ohms which is the max resistance between the middle lead and another lead. Transmitters and servos and many other analog devices use potentiometers to determine position. Servos usually use a 5000 (5k) Ohm potentiometer but it varies from servo to servo and Im sure it varies from transmitter to transmitter.

A servo plug is just a small three wire plug that servos use.

Ill open the steering/pedal set I have tomorrow if I remember and let you know a wiring diagram and how it works. The wiring diagram will be probably be different than other steering sets but it will give you an idea of how it works, and if all else fails Im sure you could find a set like mine somewhere second hand for cheap.