A network has nodes and connections between them. Since not all connections between things are the same, in the model builder you can define different "connection types". The Getting Started view that you encountered on the homepage of your initial Studio or City is based on a model that has two different connection types:
As you can see above, a connection type can be defined between two different element types (e.g. explainers -involve-> concepts), or between an element type and itself (e.g. concepts -related-> concepts). You define a connection type in the model builder by simply dragging from the arrow of one element type to the circle of another:
Many connections can be created from just a few connection types, and those connections can be visually differentiated in the views you create. In the Getting Started view the connections between explainers and concepts are grey while the connections between related concepts are green:
After you define a connection type between element types, the system will automatically create a form that allows you to enter these connections as you add data. For example, if you click the "Add" icon under the explainer element type, you'll get a form that allows you to not only create a new explainer element, but also connect that explainer to concepts, and create new concepts in the process too:
You can rename the fields associated with the connection types, and add rules about what type of connections are allowed, by changing how the element types involved in the connection are defined. See the info about element types for more details on changing element type definitions.