Each Port has a Data Type which defines what edges can be connected to it. Each data type has an associated color for identifying its type.
Only one edge can be connected to any input Port but multiple edges can be connected to an output Port.
You can open a contextual Create Node Menu by dragging an edge from a Port with left mouse button and releasing it in an empty area of the workspace.
Each Input Port, a Port on the left side of a node implying that it is for inputting data into the node, has a Default Input. This appears as a small field connected to the Port when there is no edge connected. This field will display an input for the ports data type unless the Port has a Port Binding. If a Port does have a port binding the default input field might display a special field, such as a dropdown for selecting UV channels, or just a label to help you understand the intended input, such as coordinate space labels for geometry data.