Pots are easy.
The black resistive element is a resistor.
5v on the top tab and ground (0v) on the bottom tab allows you to select a voltage (a.k.a. "potential") between those values.
A wiper arm slides over (wipes across) the surface of the board, making contact between the silver ring in the center and a point on the resistive element.
The contact point is the bump on the right side of the wiper.
When you center the knob, the arm makes contact at the green bar, giving you 2.5 volts on the wiper. (assuming it is a linear pot)
When you turn the knob to the blue bar, you get 4.5 volts on the wiper.
If you reverse the 5v and Gnd tabs, green will still give you 2.5v and blue will give you 0.5v. (reverses the axis)

Wiring pots to the A-Pac:
http://www.ultimarc.com/a-pac.html
Scott