dongun - the CUP system measures chamber pressure, just as you would suspect. The values are derived by crushing a copper "crusher" rod (or lead in some cases)between a piston mounted in a barrels chamber and an external anvil. The more it is crushed, the higher the CUP (or LUP).
CUP values really don't translate to PSI values with a single multiplication factor. As Paul said, one caliber that gets 50K CUP may get 60K PSI (e.g., 280 Rem.). But some other cartridges actually have equal or lower PSI compared to the CUP. The majority of measured PSI values are higher than the CUP values. But again, not in any particular ratio.
Of all the commonly available reloading manuals, Lyman's has the best explanation of CUP and the method used.