it would be the minimum fuel in tanks that on landing would not havee the min fuel lights coming on or having to do a flaps 20 landing in case of a go around.
Fuel lights??
Anyhow my current FCOM for the 777 says you will get the "Fuel Qty Low" EICAS message if the quantity of fuel in either left or right main tank is "low", theres no numeric amount specified....If memory and old course notes serve me correctly the trigger is less than approximately 2000 kg of fuel in either left or right tank..
Since the 747 has been mentioned when I first operated the classic 747 the company decreed min fuel load was 40 tonnes ( i.e. tank to engine feed, 10 tonnes per main tank) because of supposed worries about fuel pumps unporting etc....over the years it became apparent that wasn't a problem and on both the 747 and then the 744 we carried flight plan fuel'ish for departure, which made for fun on 30 minute or less "shuttle" sectors.