A medicine cat has the power to heal sick cats and the power to read and interpret omens from StarClan. They don't really have magic powers, but it's more of a rank privledge.
When they are born, they are either destined to become a medicine cat, like Yellowfang, or they take much interest in being a medicine cat. If StarClan approves of the official medicine cat's choice, then they will come to the apprentice in their dreams and such. -x Mossybird x-