[pjmlp]

It means you can be sure that the APIs described here exist, with at least the set of specified arguments and behaviors.

http://www.opengroup.org/standards/unix

For example, http://pubs.opengroup.org/onlinepubs/9699919799/

However, just like C, any certified implementation is free to add extra behaviors and there are certain parts that are actually implementation specific, like how signal handlers behave in certain situations.