Can someone explain what dup() in C does?

Question

I know that dup, dup2, dup3 \"create a copy of the file descriptor oldfd\"(from man pages). However I can\'t digest it.

As I know file descriptors are just

Answer 1

see this page, stdout can be aliased as dup(1)...

Answer 2

Just a tip about "duplicating standard output".

On some Unix Systems (but not GNU/Linux)

fd = open("/dev/fd/1", O_WRONLY);

it is equivalent to:

fd = dup(1);