The term potentiometer comes from the simple circuit shown below.
A voltage is placed across the outer, fixed terminals and taken from ground and the middle terminal which is connected to a wiper (a movable arm on top of the resistor). At J1, the output terminals, you read with your meter a voltage from zero to the battery voltage (here 4.5 volts) which changes as you turn the shaft of the potentiometer. Potential is another name for voltage, and meter means measure - hence a potentiometer is a way of measuring/changing a voltage.
This guide will help you understand the different types of potentiometers and how you may want to use them in your projects.
While the concepts in this guide can be used with ANY microcontroller or single board computer, they will be demonstrated using the Circuit Playground Express.
If you'd like to work with a slide potentiometer, this 3.5 cm 10K linear model is nice. As usual with potentiometers and resistors, there are many types. This one is $1.29/each in single quantities at the time this guide was written from digikey.com.