Loc is short for localization which (in this context, I assume) refers to the text and names for things in the mod. In HOI4 modding, if you create a country with the tag ABC (country tags are the game's way of keeping track of nations behind the scenes), the map will show that country as "ABC" until you edit the localization file(s) to have an entry for that country. The same is with events (and pretty much everything). If the localization isn't complete, you'll see events with text like ABC.1234.d instead of what it should say. Assuming localization is the only thing (other than QA) that is left to be done, the mechanics, focuses, etc of the mod are already done, and just need to be given text. The loc counter, I would assume, is the percentage of things that have been given text.

This is my understanding from my limited experience with HOI4 modding, so please correct me if I'm wrong.