At a temperature equal or greater than the critical temperature, no matter how much pressure you applied, you will never get a condensed(liquid) phase. That's the physical meaning. Mathematically, I am not sure if P-T curve simply vanishes or it abruptly approaches zero. I'd go for the first one, though.( They are made considering dP/dT=Δ_{r}H/RT^{2}, which cannot raise to infinity , for the function to descend steeply)