One of the new features in Windows 10 is the Photos app, which obviously shows photos stored on your computer. With the recent Windows 10 Fall Creator's Update (ugh, I hate that name), the Photos app got a major upgrade, with the ability to create videos with 3D effects/music/text. It's like a modern successor to Windows Movie Maker.
Microsoft is working on additional improvements to Photos, including a companion app for iOS and Android. Italian blog Aggiornamentilumia discovered some hidden features in the latest Windows 10 Insider Build, including the above dialog explaining how the mobile app would work. This will apparently allow users to quickly transfer photos and video over a Wi-Fi connection.
The companion app hasn't showed up on the Play Store (or iOS App Store) yet. When it does, it will probably only work with Windows 10 Insider Builds until the new Photos app is released to the main public branch.
- The Verge