This is based on an ingenious recalibration of the conventional 0 - 10 scale so that 5 is the equilibrium point at which you would just have sex with a person, but not really be enthusiastic about it. The magnitude of ratings above 5 denotes how badly you want to bone, and the magnitude of ratings below 5 denotes how vehemently you'd avoid boning.
An alternate definition of a rating of 5 useful for males is describing someone which you would neither thrust into nor attempt to remove from your penis.
Recognized antonyms: Super-5, Supra-5
"Eh, I don't really care, shes exactly a 5. I'd let her climb my dick, I guess."
"I'd pay $100 to put my dick in her hair. She's a 9."