Understanding C built-in library function implementations

So I was going through K&R second edition doing the exercises. Feeling pretty confident after doing few exercises I thought I'd check the actual implementations of these functions. It was then my confidence fled the scene. I could not understand any of it.

For example I check the getchar():

Here is the prototype in libio/stdio.h

extern int getchar (void);

So I follow it through it and gets this:

__STDIO_INLINE int
getchar (void)
{
  return _IO_getc (stdin);
}

Again I follow it to the libio/getc.c:

int
_IO_getc (fp)
     FILE *fp;
{
  int result;
  CHECK_FILE (fp, EOF);
  _IO_acquire_lock (fp);
  result = _IO_getc_unlocked (fp);
  _IO_release_lock (fp);
  return result;
}

And I'm taken to another header file libio/libio.h, which is pretty cryptic:

#define _IO_getc_unlocked(_fp) \
       (_IO_BE ((_fp)->_IO_read_ptr >= (_fp)->_IO_read_end, 0) \
    ? __uflow (_fp) : *(unsigned char *) (_fp)->_IO_read_ptr++)

Which is where I finally ended my journey.

My question is pretty broad. What does all this mean? I could not for the life of me figure out anything logical out of it by looking at the code. Looks like a bunch of codes abstracted away layers after layer.

More importantly when does it really get the character from stdin