CUP stands for copper units of pressure. It is one of the two methods used to measure the chamber pressure of a round while doing the development of a load for a load manual. All cartridges have a maximum chamber pressure as listed by SAAMI. The maximum load in the reloading manual must be below the maximum chamber pressure.
The research labs that do the pressure testing use a universal reciever that has a gas port that directs to another chamber. In this second chamber they place a copper disk specifically made for measuring pressures. This copper disk will compress a certain, predictable amount for a given pressure. By measuring the disk after the cartridge has been fired they can tell what the chamber pressure was.
The second way to measure chamber pressure is to use an electronic sensor (I think its actually a piezo electric sensor). Using this method allows the factory to actually map the pressure curve of the cartridge. In some reloading manuals it is noted which method was used to measure the pressure.
As far as those listed pressures concerns my reloading, it really doesn't. That's what the manual is for. As long as I am still within their load data I know my pressures are safe (but you should still watch for pressure signs, especially during load development). I'm a lot more concerned with bullet velocity, extreme spread, and consistency of the ammunition and with finding the best load within their data.
Note that many things that you do will change the maximum pressure of the load, things like loading the bullet jammed into the lands, overcrimping, not trimming and allowing the case to reach the end of the chamber and pinch the bullet, etc... Any time you change something about your load you need to cut the powder charge down a bit (10%) and work back up while watching for pressure signs. That part is very important.
Owner of & Chief Instructor for The Southern Marksman, LLC
NRA Life Member, NRA CRSO, Multi-Discipline NRA Certified Instructor