Geopotential height expresses the altitude at which a given isobaric surface is found. The most used in forecasting is 500 hPa (~5,500 m), where troughs (low values) indicate cold air and ridges (high values) warm air.
500 hPa geopotential height charts are meteorologists' quintessential "weather map": they show Rossby wave flow, jet stream position, and allow predicting the movement of surface lows and highs.