"Leopard can compile and run all your existing UNIX code."

It doesn't really mean anything more than some programs written for other unix certified OSs will work nicely with each other. There aren't very many that are certified to begin with though...only a handful, Apple is the fourth entity and Leopard the eighth OS, but there are plenty of uncertified OSs out there that are relatively standards-compliant, like some linux distros. Explicitly I suppose, it means any program written with the SUS in mind will work across similar OSs, and that if you write a program with the standards in mind it will also work similarly across those OSs. That is all. (even though that's big)

This doesn't mean any and every single program will function as intended, because there's so many other things that the program will need to function that the OS might not have. Just the ones following the standard.

That being said, it's interesting how only the intel version of Leopard was certified, and not the PPC...