Familiars are magic born animals able to switch from their natural animal form to a human form with animal attributes, Jim and Claire each have a curse on them that keeps them as cats and prevents them from being able to talk. Toby’s been trying to figure out how to break their curses and only finally figures it out when he encounters Merlin.
Jim’s curse was the easier of the two to break, once it’s broken, he’s able to live as a normal familiar instead of a cat with a bit of magic. Claire’s curse was more difficult and when Toby finally breaks her curse, they realize she wasn’t actually a familiar, but a witch who was trapped in the form of a cat.