Models are structural representations of your Sitecore templates, fields, folders, relationships, dependencies, diagrams and everything else that you see in the output documentation. Models can be thought of as the object representation of your architecture and documentation.
The most important thing to remember about models is that everything you can see in the documentation has a model. The model holds the data for everything that you see, whereas the views, are the actual, visual entity.
In the SitecoreDXG-generated output documentation, you can find all of your models in the navigation tree in the left-pane of the page. This tree should be somewhat similar to the content tree for your templates in Sitecore, with a few extras, like diagrams and relationships.