What is a flag event?

The term "flag event" refers to a type of step that can be incorporated into your automation.

The Flag Event allows you to split your journey path based on a previous step, previous action taken, demographic field, or category membership, or a combination of factors. 

For example, evaluate if an email was opened, a link was clicked, a subscriber belongs to a specific category, then send the contact through a separate branch of the workflow based on that information.

Flag Events, along with "Wait for Event", are the steps that allow your workflow to react to subscriber/recipient behavior and demographics.