This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
Guest relations cast members at Disneyland Resort wear plaid vests in a red and blue pattern. If you hear other guests refer to “a plaid” or “a plaid vest,” they might be using the phrase to refer to a person working in guest services, and they are likely referring specifically to a cast member leading a VIP tour or experience.