On a Unix system, you can use the man (manual) pages for any standard C library function (it may also tell you about functionality specific to that flavor of Unix, though, and not tell you that it’s non-standard), for example, the command “man fgets” or “man 3 exit” at the command line. This is the way these were supposed to be looked up pretty early on. However, we don’t all use Unix systems, and some of us are not familiar with the command line. In fact, while I recommend both of these things, the fact is that a majority of people learning programming use Windows. Usually we are not interested in the system-specific version of a function, either.
The point of telling you this is that you can go to Google and do a search like “man <function name>” (no quotes) and you will find a bare-bones explanation of what the function does, same as a Unix manual would tell you. You will probably not be given the whole picture; it may not explain things well enough for people who wouldn’t already know what it does, and it will often not even tell you whether it’s even a good idea to use that function! But if you are truly lost, it can give you enough of a description as to what you should look up next.
Constants and macros like EXIT_* would not generally be described in man pages, and for those you should consult the relevant header files.