A model is a created construct to help us understand real-world systems. Models, typically, have an information input, an information processor, and an output of expected results. A transportation model, specifically, typically uses a transportation system, plus location variables, to understand traffic flows.
An example of a common transportation model would be a Travel Demand Model (TDM). A TDM is used to forecast traffic future flows on a transportation system. This model's input would include a transportation network, population information, employment, households, transit fares, etc. It would be processed using modeling software, and it would output maps of future transportation systems with traffic volumes on those roadway segments.