A daily average background X-ray flux in the 1 to 8 angstrom range. It is a midday minimum designed to reduce the effects of FLAREs.
ON-LINE GLOSSARY OF SOLAR-TERRESTRIAL TERMS of National Geophysics Data Center

This represents the average background x-ray flux as measured on the primary GOES satellite. This value basically represents the amount of x-ray radiation that is being received at the Earth by the Sun. Generally, active regions emit more x-ray radiation than non-active solar regions. Therefore, this value can be of use in determining the overall state of the solar hemisphere facing the Earth.

This value is also useful for propagation prediction models (ie. PROPHET models), since ionospheric layer ionization is closely correlated with the background X-ray flux. This flux is stated using the same classification scheme for x-ray flares (given below).

Mr. Vaino K. Lehtoranta