Here, it shows "C22": [Name of opening]. This is from an online service, and I was confused what this is. At first, I thought it was for the specific sites database, but I have seen the letters and numbers used with the opening name on chess.stackexchange.com too. What is it?
The Encyclopedia of Chess Openings (or ECO) is a classification system for the opening moves in chess.
Instead of the traditional names for the openings, ECO has developed a coding system that has also been adopted by other chess publications. There are five main categories, "A" to "E", each of which is divided into 100 subcategories.
Here is more info and related questions in case you are interested:
And this is the exact same code that you mentioned, covered in another source: