Data Events allow users to perform specific actions when certain events are triggered. Actions can include things like custom alerts and validation messages, setting field values, choices, labels, descriptions, requirement & visibility settings, HTTP requests and more. Event triggers include things like record loading, editing, validating, saving, value changing, and more.
To access the data event builder click on the Data Events button in the right side of the app designer within the app settings section. Just above the save buttons.