What is really core mods? how do you make a mod into a core mod?

I've heard of core mods a lot of times, but I must admit I never figured out what they mean from a coders view.

As it is mentioned that a mod "has to be a coremod" inn order to do somethings, like methods and such I'm wondering how you mark your mod as a Core Mod? For me I only thought of core mods as the mod containing the common logic base for a group of one's mods but I've been proven wrong by the answers in other threads, so now I'm pretty clueless

I tried googling it but that of course only lead to people mentioning the core mods they have made and searching on these boards made it show up every post with an error log. And the wiki was blank after a search on "core". So could someone enlighten me on how this works because for me it seems weird that you should be unable to use some functionality without being core unless you need to extend or implement something for access, so what do I have to do and what is this?