With the app, you open up a video that you have already taken and stored on your phone, then use the app to compress it.
Once compressed you can then save it into a different folder in your phone which the app sets up for you, then upload that version of the video, not the original.
The difference can be as much as 25% smaller file sizes, if not more in some cases, which means the video will upload a lot quicker, take up less storage space, and render faster in browsers.