Bass side and treble side are references to the orientation of the tuner. More specifically the side of the headstock the tuning machine should be mounted on. A bass side tuning machine has the tuning shaft protruding from the right side of the tuner and a treble side tuner has the shaft protruding from the left side.
For example, a right handed jazz bass would have all tuners on the bass side. A reverse headstock or left handed jazz bass would have all treble side tuners.
