ULDs contain air freight cargo.
A ULD (unit load device) is a device used to move cargo being shipped as air freight. More specifically, a ULD is used to consolidate cargo in order to assist with the loading process onto an airplane.
ULDs come in two forms: pallets and containers.
Air cargo can be tendered loose to an airline, and the airline will load the air shipment into a ULD with other shipments.
Shippers can also book a ULD container and load the ULD themselves. Then a trucker at destination can pick up the entire ULD and deliver it to a warehouse to be deconsolidated.