Bobtail describes a tractor (the truck cab and engine unit) running down the road without a trailer hooked up. You’ll hear it when a driver is heading to a yard to pick up a trailer, or driving back after dropping one off.
Bobtailing earns no revenue — there’s no freight moving — so dispatchers try to minimize how far and how often a truck runs bobtail. A tractor also handles braking and traction differently without a trailer’s weight behind it, which is worth keeping in mind on slick roads.