All content is automatically saved in the app's library. To save videos to your phone you can press "select" in the top right corner of the library then choose the video you want to import to your phone and press download. The video should now be in your iPhone's library.
If you plan on deleting the app, importing content directly to your phone will save the clips. You can also upload clips from your phone's library to the app after reinstalling. If you uninstall the app and don't save your clips, they will be deleted as well.