What does "reefer" signify in Reynolds' books?

REEFER is a contraction of "Refrigerated Container" and will keep the contents at a pre-configured temperature, often -18 degrees C for "deep frozen" items like meats. Items like fruits need to be chilled to perhaps +4 degrees C, and not frozen.

The bodywork is lined with extra insulation, and there may be humidity control systems to protect the contents. A nominal "20 foot container" or TEU will have slightly less internal capacity when it is refrigerated compared to a normal sized container due to insulation and motor. The motor is outside the storage space, its in a separate protected area at one end of the container.

Here are some stacked reefers, mixed in with regular shipping containers (also known as "sea cans")

The white ones have a motor pack visible on the end, which are connected to an all-weather power socket. This is charged as an additional cost when booking space on a vessel. Trucks also have power ports to run the motor.

The motor runs a compressor which is connected to an evaporator and condenser. There are often fans to increase airflow over both.

In the context of this story, a reefer is an ice-box or ice-coffin for preserving a living person for a long voyage. The Reefer is probably a "plug and play" unit with a lot of self-contained systems, requiring only power to function. It may have an internal power source as backup for some amount of time, or it might be independent of ship's power.

Refrigerated cargo ships have been known as "reefers" throughout their 150-year history. They were initially used for transporting frozen meat, but a major specialist cargo was bananas. Since bananas have a relatively low density, the banana reefers soon began to carry passengers as well as the cargo, and by 1900 they were operating somewhat like modern cruise ships. The US-based United Fruit Company, which was the biggest shipper of bananas worldwide, had one of the largest non-military shipping fleets in the world in the early 20th century.

Many of the "banana boats" were used as troop carriers in WWI and WWII.

Modern shipping often uses refrigerated containers (powered by the ship's system) instead of refrigerated bulk cargo, but the "reefer" designation is still in use.