Mathematical models describe a real-life phenomenon, using mathematical language. The model will not exactly be like the reality, but can be used to learn from and help us make decisions.
One way to mitigate the consequences of drivers' selfish behavior, is by introducing tolls (or taxes) on certain parts of the road network. This is also known as road pricing.
Manufacturing plants convert raw material into a final product. Think of cars, where the production line consists of a large number of phases to put all the different parts together into a working car. Big machines in such a plant perform the processing steps in different phases, which often have to be done in a specific order.
In network congestion models, we make some simplifying assumptions that make our life easier. In a large-scale system, each individual driver contributes a tiny amount to congestion, if we assume that every car controls the same amount of traffic.
