I always see a sheep sign in multiple buffy episodes. Does anyone know what the sign means?

In earlier seasons it is pictured in both Willow and Xanders room as well as on the high school lockers. After watching the last episode of season 4 where the first slayer haunts their dreams I again saw it as Xander goes to meet Tara and Willow in the back of his ice cream truck and emerges in his basement.