I found this site which goes into NaCl (sodium chloride or simply, salt): http://www.tis-gdv.de/tis_e/ware/gewuerze/salz/salz.htm

Apparently at 75% relative humidity, NaCl crystals begin to disolve. Thus, by adding water to NaCl to its saturation point, you are basically replicating this in a controlled environment allowing you to properly calibrate your hygrometer.