The Locked Folder is a password-protected place where you can hide photos and videos from your library. One of the key features of this folder was that the media placed in it are not backed up to the cloud.
And there have been cases when many users, unaware of this fact (although it is clearly written about it), have simply lost their hidden photos and videos when resetting their devices.
Fortunately, this may soon change, as the latest app update, ver. 6.23, has added a hidden settings page that is responsible for backing up photos and videos from your Locked Folder to the cloud.