What does it mean to write to stdout in C?

stdout stands for standard output stream and it is a stream which is made available to your program by the operating system itself. It is already available to your program from the beginning together with stdin and stderr.

What they point to (or from) can be anything. Actually, the stream just provides your program an object that can be used as an interface to send or retrieve data. By default it is usually the terminal but it can be redirected wherever you want: a file, to a pipe going to another process and so on.