WordPress “core” is what we call the project as a whole. When you download and install WordPress, you’re using its “core.” Core is made up of hundreds of functions, classes, and third-party libraries, and working on them requires a careful attention to detail.
In most cases, proposing a modification to core is a lengthy and complex process. Some are aesthetic, spiritual, functional, or may have economic ramifications for plugin or theme authors. They typically involve considerable research, thought, modeling, discussion, historical knowledge, and several iterations.