A reefer is a refrigerated trailer — the name is short for “refrigeration unit.” It has its own cooling system that holds the cargo at a set temperature for the whole trip, which lets carriers move perishable freight like produce, meat, dairy, and medicine.
Reefer freight comes with extra responsibility: the temperature has to be set correctly and monitored, and a failure can spoil an entire load. That added care is why reefer loads often pay more than the same run in a plain dry van.