You see these out in the Western USA, Canada, Australia (where it is most common) and other desolate locations. Basically it is when the truck is connected to multiple trailers. The longest train was in Australia around 1993 with 21 trailers extending 1,033 feet (315 meters)!
Typically in the USA commercial trucks are limited to two trailers. However, there are some states that will allow 3. These typically are Oregon, Idaho, and Montana, but there are exceptions. Of course they must be marked and every safety precaution must be observed.
These can be very dangerous if not handled properly. Often an inexperienced driver will have difficultly pulling these. The main issue isn’t in the actual maneuvering or handling, but getting stopped! Due to the configuration and weight distribution the brakes can lock up easier on slick roads.
