It means "the aircraft design met applicable requirements at the time it was certified." It's similar to how I can continue to drive an old car that doesn't ha

It means "the aircraft design met applicable requirements at the time it was certified." It's similar to how I can continue to drive an old car that doesn't have ABS, since ABS wasn't required or invented when the car was built. The difference of course is with cars we update the rules and apply them to all new cars as of a certain date. We don't allow someone to keep making cars using a design from 1967, at least not without including current safety features.