It means nothing beyond the fact that they want that name to stand out. It's just something their agent negotiated that sets their name apart from other names that are credited. There isn't rules to it or anything. They asked for it and had enough pull to get it.

One case where it might sort of mean something though. Sometimes there will be multiple really big stars. Not everyone can have their name go first, so one of them will instead negotiate to get the final spot with the "and" since they can't get the first billing spot. That's more common in films though than shows.