The altitude is computed from the local barometric pressure using the 1976 standard earth atmosphere formula. This formula, apparently works best at about 45 deg latitude, so if you are near the equator, like I am, then the default average elevation will be slightly offset from reality.
The observed value altitude can be calibrated, when the elevation at a location is already known. The approximate ground level elevation of where you are can be got from any mapping app.
Since the barometric pressure varies with the weather, this application may not give accurate results over a long period of time (say several hours), but the relative altitude/elevation is quite accurate for periods of ten's of minutes to less than an hour.
The rate of change of altitude is computed, which is useful to see how fast an elevator goes, or perhaps for skydivers, paragliders or birds.
Important note on Data Recording: Since the Android O/S is not meant to support an App running continuously as a background process, the only reliable way to continuously record data with this app, is to ensure that it is always a foreground process.
Keywords: Barometer Altimeter Pressure