This program is supposed to wait for user input for 2.5 secs before printing time out. or print "A key was pressed!" if user presses a key within 2.5 secs.
select() also returns the number of file descriptors signalled, so you don't need to check the descriptor in this case, you can just check what select() returns, zero for timeout, one for stdin being available.
I've just noticed that you're running on Windows. I don't think select() works on file descriptors on Windows, it only works on sockets. On Unix (and Linux), a socket is a file descriptor. But other OSs implement sockets seperately, outside of the kernel; in which case a socket and file descriptor are different things.