A "jumbo" truck or tractor-trailer is a type of truck or tractor-trailer built to carry heavier loads than a conventional truck. These vehicles often have extra axles and wheels, allowing them to carry bigger loads while remaining below legal weight limitations. They're frequently utilized to transport huge or overweight items like heavy equipment, large industrial machines, and structural steel. They are also frequently used to transport heavy things like construction materials, wind turbine blades, or massive tanks. 

The name "jumbo" refers to the size of the cargo that the vehicle can convey, which is greater than that of a conventional truck.