When an action runs, it completes its job and passes data to the next action in your workflow in what we call an event. The number of events emitted by a particular action is shown in a pill in the top right of each action. For example, here we see that each action in this story for sending daily jokes in an email have 3 emitted events each.
Under the events tab you'll be able to delete or re-emit events, as well as view the details of each event:
Events emitted from all actions upstream will be contained in the receiving action
Events represent the structured data sent between actions. Events are JSON objects making them flexible and easy to read. All events are timestamped and "write-once", providing an audit trail and making them an ideal basis for the calculation of key performance metrics.