It's simply the following voltage levels:
-THe lowest input voltage that is considered a LO (is usually Vss/Vee/ground)
-THe highest input voltage that is considered a LO.
-THe lowest input voltage that is considered a HI.
-THe highest input voltage that is considered a HI (is usually +Vdd, or Vcc)
-THe lowest voltage that may be outputted when trying to output a LO (is usually Vss/Vee/ground)
-THe highest input voltage that is considered a LO.
-THe lowest input voltage that is considered a HI.
-THe highest input voltage that is considered a HI (usually Vdd or Vss)