It means a service that has no passengers. It’s to move stock around the system to prepare for a revenue (passenger) service later in the day.
Pretty much what the posters above said. I guess it's also known as an empty stock move on other routes? Basically what usually causes some routes to have these long "deadhead" runs is that the big yard/maintenance facility is pretty far from the big terminal where the services starts or finishes. It's not ideal for a company that aims to maximize revenue b/c in practice you're burning miles while not producing revenue (aka dead mileage). Probably a bigger issue for private bus companies tbh
Deadheading is a generic term in the transportation industry as a whole for any vehicle/employee movement that isn't for the purpose of generating revenue. For instance, airline crew will often "deadhead" on a flight as regular passengers (traditionally for free) to get to/from their operating airport (bus and train drivers can do the same too if their runs don't start from a depot). In trucking, it is when you need to drive all the way back to your base but don't have any return cargo ("backhaul") to offset the running costs.
