Hello, 

Well I would say a FlowAction is a task for a user. Once that task is complete the case flow will continue. 

Please find the below article which might be helpful.

https://community.pega.com/sites/default/files/help_v63sp1/Content/definitions/f/flowaction.htm

 Some sequence of events to be performed to complete a business process. Flow action is a process rule which allows user to perform an action on the assignment and it moves the case from current assignment to the next.

Flow actions are of two types:

• A local flow action permits users at runtime to update, but not complete, an assignment. Local flow actions always are optional. 
• At runtime, users choose a connector flow action, complete the assignment, and advances the work object along the connector to the next task. A line exits from an assignment shape and ends at the next shape in the flow