Google says the app stabilizes the camera movement of a Live Photo for a steadier shot with less visible shakiness. Once downloaded, the app doesn't need internet access to work, just access to your iPhone's photos.
The app does more than just get rid of camera shake, though. It also processes the video frames to isolate the foreground from the background so that the background can be stabilized separately. That's useful when creating a GIF that loops forever, a Motion Stills option that means you can turn Live Photos into cinemagraph-like moving images.
You have to create a Live Photo using the native camera app first. Then, you use the Motion Stills app to convert the photo into a steady GIF (or video clip, if long) that can be shared via text message, social media, Slack, etc. The app also allows you to create short movies by stitching together clips.