Congratulations ... now the hard thing is dealing with the addiction
When adjusting the altimeter he was changing from QNH (the height above sea level, used for local area flights because that's how the chart spot heights etc. are marked) to QFE (the height above the ground at the airfield datum). It's normal
Strictly speaking, the reading using QNH is called altitude while the one using QFE is called height. There's a third, Flight Level, using a pressure setting of 1013.25mb.
(The instructor would have made a radio call for rejoin instructions just before the adjustment, during which they would have told him things like runway in use, circuit direction and QFE)