This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
Safety is a foundational principle for Disneyland Resort, so attractions have appropriate seating and restraint systems, and they also have ways for cast members to verify that the restraints are working properly. On attractions with seatbelt-style restraints, guests buckle their own restraints. In the strap near each buckle is a short yellow tab. Cast members will check that everyone has safely buckled themselves by then walking by and asking all riders to pull on this yellow tab. If the belts remain secure, the cast member will confirm that the guests are ready to ride.