Power only means that just the tractor and the driver are needed, not the trailer. Shippers that own or lease their own trailers can hire these tractor and driver sets (called power units) to haul their freight.
Many people make money with their pickup truck by delivering power only pickup truck loads. The term “power only” refers to the fact that these carriers often travel with just a pickup truck, or perhaps a pickup and a trailer, rather than a massive trailer equipped for heavy haul loads.
Running power only can help you increase your efficiency as a carrier because you don't have to spend a lot of time matching specific types of loads with equipment. Perhaps the greatest advantage of power only trucking is that you/your drivers don't have to deal with long waits at shipper and receiver facilities.