A truckload is as much as one truck can hold. It is a unit of amount, the way *a cupful* or *a busload* is: "a truckload of sand", "three truckloads of furniture". Used literally it is genuinely useful for anyone ordering or delivering anything — builders, farmers, shops all count in truckloads because a truckload is how goods actually arrive. But English loves turning big containers into words for "a lot", and *truckload* has gone that way too. "A truckload of homework", "a truckload of money", "a truckload of problems" — none of these involve a truck. In this exaggerated use it is informal, spoken, and a bit humorous; it belongs with *a mountain of*, *a ton of*, *loads of*. American speakers say it more than British ones. Small point of grammar: it is a countable noun, so *truckloads* is fine, and it takes *of* before the thing being measured. Also compare the family it sits in — *carload*, *busload*, *boatload*, *planeload*, *cartload*. Any vehicle plus *-load* can be pressed into service this way, and English speakers do invent them freely.