Plain English Meaning
Extra fuel you must have on board, on top of what it takes to fly to where you are going. The rules set a minimum so you still have flying time left if something goes wrong on arrival.
Full Definition
The minimum quantity of usable fuel an aircraft must carry beyond the amount needed to reach the intended destination, as required by regulation. Under 14 CFR 91.151, day VFR flights require fuel to fly to the first point of intended landing plus 30 minutes at normal cruise; night VFR requires 45 minutes. IFR fuel requirements under 14 CFR 91.167 require fuel to reach the destination, then the alternate (if required), plus 45 minutes at normal cruise.