It is a bus that mentally disabled people are transported on. The white bus is commonly seen full of people driving around Glasgow. It is sometimes the object of ridicule, for example, it can be used to insult someone.
1. "Look, there's a white bus coming up the road. That's a shame."
2."Shut it, you get the white bus to school."
*On seeing a white bus driving away*: "Look Josh, you missed your bus."