In cycling, cadence refers to the rate at which you're pedaling (turning the the bike's crank shaft). Cadence is usually expressed in revolutions per minute (RPM).
Some GPS devices can gather cadence data from a separate sensor mounted on the bike, allowing you to correlate cadence with speed, elevation change, and location data.
GPS models that can gather cadence data include:
Many non-GPS devices can gather cadence data, including some heart rate monitors made by Polar.