Microsoft FLOW is an online workflow service provided by Microsoft as part of Office 365. You can perform various actions on different events using FLOW. For example - do something when the mail comes in my mailbox, do something when an item is added in SharePoint list, inform everyone when a new post is added in Instagram, etc.

Well, having said that, there might be a need to trigger your flow from PowerApps app (which is calling On-Demand). In this video, we will see how to create a simple FLOW and how to add it in the PowerApps app and run it from there.