Conference of the Parties. The Parties are the 198 countries that have ratified the 1992 UN Framework Convention on Climate Change, and the COP is that treaty’s decision-making body. COP31 is the thirty-first annual meeting.
The name is bureaucratic and it hides something simple. A treaty needs a body that meets, reviews whether it is working and decides what to do next. The Convention says in its own text that the Parties shall meet every year unless they decide otherwise, and they have, since Berlin in 1995.
Almost every country on earth is a Party, which is why a COP is one of the few rooms where nearly all of them sit together on the same subject. It is also why decisions are slow: the process works by consensus, so a single country can hold up an outcome.
There are other COPs, confusingly. The biodiversity convention and the desertification convention hold their own. When people say COP without qualifying it, they almost always mean the climate one.