What is the meaning of the flow arrows?

  Email Parser connects one item to the next using the arrows you can see in the left panel. When an email is received Email Parser will follow these arrows and run the items one by one.   parse email flow of data to export to excel    

What happens if an item has no flow arrows pointing to it?

It will never be run. Email Parser will ignore it.

How can I change the flow arrows?

When you create a new item Email Parser automatically creates a flow arrow that comes from the immediate previous item but you can change it easily in the item settings: