The In-App delivery activity is generally used to automate sending an In-App message to a target audience calculated in the same workflow.
The recipients are defined upstream of the activity in the same workflow, via targeting activities such as queries, intersections, etc.
The message preparation is triggered according to the workflow execution parameters. From the message dashboard, you can choose to request a manual confirmation to send the message (required by default). You can start the workflow manually or place a scheduler activity in the workflow to automate execution.