What do those railroad exempt signs mean?

Buses and some other types of commercial vehicles are required to stop at all railroad crossings in order to make sure there is no train coming. This is true even at crossings that have been cut off and can no longer be approached by a train.

When there is an Exempt sign at the crossing they are not required to stop. It happens often at tracks that are cut off or abandoned thus there will be no train to stop for.