In the early history of pickups, they were rated as half-ton, three quarter ton, and one-ton. A half-ton was capable of hauling 1000lbs(450kg).
Over the decades, pickup became larger and more capable, but are still referred to as half-ton, three quarter ton, and one-ton. For instance, the 150/1500 series pickups from Ford/RAM/Chevrolet are part of the half-ton class, even though they can haul considerably more weight now. A 2025 Ford F150 in some configurations can haul 2400lbs, much more than half a ton.
The 250/2500 series pickups are still referred to as three quarter-ton. The 350/3500 series pickups are still referred to as one-ton. Again, the modern US pickups can haul significantly more than three quarter(1500lbs) or one(2000lbs) ton
It is really an outdated reference, even though it is still frequently used here in the US.