Microsoft PowerApps provides a rapid application development environment to build custom apps for your business needs. The custom apps can connect to your business data stored either in the underlying data platform (Common Data Service) or in various online and on-premises data sources. These apps have a responsive design, and can run seamlessly in browser or on mobile devices (phone or tablet).
You can create an app using PowerApps for an existing list in SharePoint Online from within PowerApps or SharePoint Online. More information: Generate a canvas app in PowerApps from a SharePoint list