Background
Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures air pressure, which decreases with increasing altitude following the barometric formula, and from the surrounding's pressure calculates and displays the corresponding altitude.
To display altitude above sea level, a pilot must recalibrate the altimeter according to the local air pressure at sea level, to take into account natural variation of pressure over time and in different regions. If this is not done, two aircraft could be flying at the same altitude even though their altimeters appear to show that they are at considerably different altitudes. This is a critical safety issue.
Flight levels solve this problem—of aircraft flying at the same altitude, though their altimeters indicate different barometric heights—by defining altitudes based on a standardised air pressure at sea-level. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure.
Read more about this topic: Flight Level
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