QFF is a Q code. It is the MSL pressure derived from the barometric pressure at the station location by calculating the weight of an imaginary air column, extending from the location to sea level, assuming the temperature and relative humidity at the location are the long term monthly mean, the temperature lapse rate is according to ISA and the relative humidity lapse rate is zero.
This is the Australian Bureau of Meteorology method; QFF calculations differ among meteorological organisations. Another definition of QFF assuming an isothermal layer at the station temperature can be found in the article Mean sea level pressure.
This is the barometric pressure at the surface reduced to MSL using the observed temperature at the surface (this assumes an isothermal layer from MSL to that surface). QFF accounts from the effect that temperature has on the pressure lapse rate and therefore the resultant calculated pressure. The range of QFF so far recorded, low pressure to high pressure of, is from 856 to 1083 hPa.