What is Host Environment?

Originally Posted by C99

int fflush(FILE *ostream);

If ostream points to an output stream or an update stream in which the
most recent operation was not input, the fflush function causes any
unwritten data for that stream to be delivered to the host environment to
be written to the file; otherwise, the behavior is undefined.

As is shown above. What is host environment?

It's a technical term from the C standard. In layman's terms, it's the operating system. Though this definition is not entirely accurate, it is sufficient for all practical purposes.