The flow of a workflow
Actions have the ability to output items and provide them to the next action in a workflow. Similarly, actions have the ability to accept input from previous actions.
Consider a workflow with a Get Upcoming Events action followed by a Show Directions action. When run, this workflow will retrieve the next event on your calendar and then open a maps app with directions to the location associated with that event.
The Get Upcoming Events action outputs calendar events, and the Show Directions action accepts a location as input. Because calendar events have a location (that is, where the event takes place) a connection is formed between the two actions. This connection is indicated by a line in the center between the actions.
Workflow uses a technology called the Content Graph to intelligently convert between different types of content, allowing these connections to occur seamlessly. For more details on the Content Graph, view this article.
If two actions are not compatible with each other, a connection will not be formed between them and the actions will be spaced apart. Even when actions are not connected, a workflow may still work as intended; the spacing simply shows that the output of the first action and the input for the following action don’t match.