In everyday terms:
QNH - Observed pressure reduced to mean sea level using the ICAO Standard Atmosphere.
QFF - Observed pressure reduced to mean sea level using actual conditions.
Of course the real meaning, as any good WOp or TAG will tell you is:
QNH - If you set the subscale of your altimeter to read .... millibars, the instrument would indicate its elevation if your aircraft were on the ground at my station at .... hours.
QFF - At .... (PLACE) the atmospheric pressure converted to mean sea level in accordance with meteorological practice is .... (OR was determined at .... hours to be) .... millibars.
Now, how about QHE4 and QCF? Two plastic peanuts for the first correct answer!