What is refrigerated on a trailer?

A refrigerated trailer is insulated and has a climate control unit affixed to the front which can either refrigerate or heat the trailer to a specified temperature. Often used for transporting perishables and temperature sensitive chemicals. Lots of companies run them.... they are the mainstay of fleets such as C.R. England, Navajo Express, Stevens Transport, etc., and many other major carriers include a refrigerated division in their fleet.