What are the rats hauling in Mrs. Frisby and the Rats of NIMH?

The rats are hauling an intricately designed pulley machine which is used to assist the main character, Mrs. Frisby, a mouse whose primary goal is to secure a new location to establish her home with her children after her son Timothy recovers from a debilitating case of pneumonia. It is imperative to note that these rats have morphed into creatures with an enhanced level of intelligence—to the degree that they are capable of orchestrating a society that bears close resemblance to modern human civilization within the developed world.
The rats who are able to establish their sophisticated way of life were initially subjects of a grand experiment in which they were able to attain the same skills that took human beings millions of years of evolution to obtain. After the rats are given a drug that alters their otherwise organic capacities, one of the rats, named Justin, is able to declare himself the leader of the group after breaking them free from their cages by reading the printed instructions.
When the rats return to their natural habitat, they grow tired of competing with their human neighbors and robbing humans of their goods that they consumed. They desire to cultivate a community of their own that is based solely on agriculture. Thus, the pulley that the rats haul illustrates the power of scientific experimental intervention and how it has caused this cluster of rats to boost their chances of survival and quality of life.