Great question @freebornangel! Its called a variometer. It tells you when you are going up or down. It really helps when you are above about 2000ft asl. (Above sea level) When you are climbing up in a thermal it beeps fast, when you are going down it makes a low pitched noise. It also has a GPS in it to tell me things like ground speed, wind direction, and above ground altitude. Mine is a Flymaster. They are about $400 dollars.