Hi, Cadence is the rate at which you spin the wheels, regardless of which gear you are in. Typically measured in rpm.
Depending upon the spec of Garmin 500 you bought, it may have come with a cadence sensor. It fits on your pedal and is 'read' everytime it passes the Edge unit you have fitted on to your chain stay. If you don't have it, I think you will need to buy one. Good luck.
I am new to road cycling and have just purchased a garmin edge 500, good you please explain cadence and do I need to purchase a cadence sensor. Many thanks