That means that anything that the cat was born with will be covered if it hadn't been discovered before your cover started. If you know your cat has hip-problems before you get the cover, then that's not covered. If your vet discovers a congenital hip-problem after you're insurance has started (and usually that date plus at least a week or two), then that *is* covered, because you couldn't have known.
And the usual "I am not a lawyer", nor even remotely connected to insurance professionally.

Ask the insurance-company what they mean when you call up to talk insurance (always best to have a person answer directly - get it backed up in writing), and look at their web-site as well as you can get cheaper insurance via the web than the phone at times. If you mention that it is cheaper on their web-page, they may give you that bonus as well.