AmbientWeather.com: Barometer Definition.
A barometer is an instrument used to measure atmospheric pressure.
To compare pressure conditions from one location to another, meteorologists correct pressure to sea-level conditions. Because the air pressure decreases as you rise in altitude, the sea-level corrected pressure (the pressure your location would be at if located at sea-level) is generally higher than your measured pressure.
It can also be used as a tool to predict weather.