I am retrieving ADSB data from Opensky-network.org. I would like to know what the baro altitude represents, is it the height above ground level or above mean sea level ?
From the FAA's Frequently Asked Questions (FAQ) regarding ADS-B (Under the "Operations" section, 6th bullet point):
Can you please explain the altitude and velocity reports that ADS-B provides?
ADS-B reports two kinds of altitudes: barometric and geometric. Barometric or pressure altitude is the one pilots know best - this is the altitude that is displayed on the altimeter in the aircraft. Geometric altitude is calculated by GPS as the height of the aircraft above the earth ellipsoid. These two altitudes are not the same, but having both allows for applications that require one or the other as an altitude source and provides a means of verifying correct pressure altitude reporting from aircraft.
(emphasis is mine)
As noted above, the Barometric altitude is "Pressure Altitude" (defined as the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer.) Source: FAA's Pilot Handbook of Aeronautical Knowledge, page 4-4.
Unless the already corrected local pressure set in your altimeter is 29.92 in. Hg, in order to know what the actual (True) altitude above mean sea level is a correction factor would have to be applied to the ADS-B reported altitude based on the actual atmospheric pressure outside.
I don't know what post-processing is done by the opensky-network, but the altimetry transmitted by ADS-B from the aircraft is the pressure altitude. This is the pressure measured by a static port on the aircraft, converted to altitude according to the formulas or tables of the ICAO standard atmosphere.
The ICAO Standard Atmosphere is hydrostatic atmosphere model which assumes a sea level pressure of 1013.25 hPa, a sea level temperature of 15 degrees Celsius and a lapse rate of -6.5 degrees / 1000 meters (up to 11 km, then the temperature is constant up to 20 km) .
The pressure altitude is equivalent to the Flight Levels used in air traffic control. A flight level is equal to 100 ft of pressure altitude.
The pressure altitude is not equivalent to the height above ground. The aircraft's altimetry system has generaly no way of knowing where the ground is (radar altimetry is not standard on every aircraft and is not connected to ADS-B).
The pressure altitude is also not equivalent to the height above MSL, because the assumptions of the ICAO standard atmosphere are typically different from the actual atmospheric pressure distribution.
ADS-B does also transmit geometric height, which is GPS derived and is expressed with respect to the WGS84 ellipsoid.