What it means is that if you run up 1 hr on the tacho they will only charge you for 0.8 of this (48 mins) at the hourly rate.
However, the Tach runs all the time you have the engine running so you are charged for taxiing, run ups etc.
Some places charge brakes on to brakes off, but as there is no meter then this relies on accurate measurement by the pilot.
The 0.8 is approx the time you will be in the air. ie if the tach runs for 60 mins then 48 mins will be flying brakes on to brakes off.
The tach runs slower when the engine is idling/taxiing so your 12 mins could be actually be almost double that.
ZA