the sound, similar to tinkerbell-like ringing, a car makes alerting the driver that his/her seatbelt has not been fastened yet
Guy 1: "I usually don't buckle up if I know I'm only going to be the car for a second, but the damn seatbelt fairy wouldn't shut up, so I gave in"
Guy 2: "I actually don't mind it, she reminds when I'm danger."
Guy1: "True. Thanks Seatbelt Fairy!"