Flip Flop circuit - what is high in relation to?
"High" and "low" are the digital output levels.  For CMOS, these will usually be the supply and ground level, respectively.  To see what that is, read the datasheet.  Even crappy datasheets will tell you what power voltage the part needs.  The datasheet should also tell you the maximum guaranteed logic low voltage and minimum guaranteed logic high voltage, within the current source/sink range also defined in the datasheet.