What is an encoder?

[kgzak]

Simply speaking it allows you to see how far or how fast something has rotated.

inside there is a light or laser that shines on or through a disk that spins with whatever it is that you want to measure. That disk will either have small holes or lines that trigger signals that are then sent back to the controller (most used in FIRST have an A and a B signal). The higher the counts per resolution the more precise the encoder is.

Hope that gives you a basic understanding.