Comment by tomphoolery

It just means that the operating system ships with the properly-licensed UNIX software binaries and libraries, rather than the GNU free software replacements for said software. If you look in the man pages for any shell command OS X ships with, you'll see it's from the "BSD General Commands Manual". On Linux systems, this originates from GNU. Your example of bash is actually not part of that distribution, it's something additional OS X ships with just for userland purposes. I suppose the point of being a "certified UNIX distribution" is just so they can put the trademark on their website because it looks pretty.