Most all data in Quill flows left to right, moving from source nodes to eventually view properties. When data moves the other direction, from the UI to whoever is providing the data (e.g. code), it's referred to as backflow.
For example, if you have a text property as an input the view will display whatever value the property is connected to. But, if you have the editable property set the user can make changes to the text. When a change is made, the new value is sent back in the other direction, and the data provider is responsible for handling the change. A Data Node will store the new value, a Source Node will send an INK_DATA_EVENT_CHANGE event, etc.