i don't understand it says that it is are they but said the correct answer is they are please reply thanks
Not really. It would be acceptable if you added a noun at the end of your sentence, e.g. animals - Jsou to stará zvířata. This would be correct.
It's a substitute for any word that is a neuter plural. For example města ('cities'), zvířata ('animals'), kuřata ('chickens'), kůzlata ('goatlings', 'goat kids') etc.