A transition altitude is an altitude where the atmospheric pressure bug on the aircraft is reset from standard pressure (QNH1013 hectopascals or altimeter 29.92 inches of mercury) to the local atmospheric pressure.

Phraseology is changed too.

Example:

London Heathrow has a transition altitude of 6000ft. An aircraft, in this example, SHT3L is at FL240.
SHT3L d 120

SHT3L descend to flight level 120
(12000ft is higher than the transition altitude, so you say flight level)

SHT3L d 60

SHT3L descend to 6000 feet, QNH999 hectopascals
(6000ft is the transition altitude, a lower altitude would also say feet)

Is this something worth implementing if weather or METAR retrieval is implemented?