by Tim Hunt.
Well, your wording is not clear. A course context does not ask anything. It is a thing, not a question.
If you understand object-oriented programming, then one way to understand it is that a context is like a base class for "subset of a Moodle site", with subclasses like Course, module, block, system, etc.
Another way to think about it is like a file system. You have your hard drive, which contains folders, which contains files. The system context is is like the whole hard drive. Course and course-category contexts are like folders, and module, block and user contexts are like files.
Checking a capability is like checking a file permissions on a file or a folder.