What does a COSC certification or Chronometer rating on a watch really mean?

While highly touted, a COSC certification or Chronometer rating on a mechanical (manual wind or automatic) watch really doesn't mean all that much anymore. Most any modern mechanical watch from a luxury watch brand is capable of operating at the performance level tested to receive these certifications. 

People frequently mistake a chronometer rating as an absolute guarantee of performance to a particular specification -- usually the often quoted -4 to +6 seconds per day which is actually only one of the seven performance measurements used in the test. In fact, the certification testing is nothing more than an additional quality control test that the movement in the watch passed at some time in the past, long before it was even assembled into your watch. It is not a guarantee that the watch will never deviate outside a particular range. In fact, mechanical watches are normal to deviate--that's why the chronometer rating exists in the first place. The range of these tests was established decades ago, when the average mechanical watch was nowhere near as accurate as they are today. 

Imagine getting an A+ in math in high school and then being expected to never ever make a mathematical mistake the rest of your life. That's not how performance tests work. They only certify that you have proven once to have achieved a specific measure of performance under reasonable conditions. They neither mandate that you can never vary below that level in the future, nor do they prove that someone else who did not take that same test can never meet or exceed your level of performance. But having once established that you can achieve that level, the certification gives greater public confidence that you can perform similarly well in the future. 

Same with your mechanical watch. It may have passed a test in the past, showing it was capable of getting an A+ in performance. But the actual day-to day performance of any mechanical watch--certified or not--can change a lot based on how well the watch is wound, what position it is left in overnight on your dresser, whether it takes a significant bump during the day and other factors. So do not give tremendous amount of weight to the significance of these certifications. In particular, do not assume that a specific watch is inferior because it lacks a certification--it did not take the test and fail, it simply never took this optional extra test. 

The reason that some watches are still submitted for these certifications is more as a marketing and sales tool among competing brands. The presence of a certification can give you a small measure of additional confidence in the selection of your watch, but nothing more than that. Most any mechanical watch from a premium brand, when adjusted properly, will perform to within the certification ratings.