By default, your files are only stored in the app, so you will lose your files if you delete the app or lose/break your phone.

If you have enabled automatic iCloud backup on your iPhone/iPad to backup your entire device, you may be able to restore a previous backup and get your files back. Details here:

You can also export your files from your Locker and save them however you want:

If you have Premium Pass and choose to enable cloud backup in Locker (this feature is no longer available for uploading new files), the files you choose to back up are backed up in your own iCloud storage, so that you can restore files if your phone is ever lost or broken. We are never able to see your files. Note that you must backup each file you want backed up, they are not automatically all backed up if you enable cloud backup. This option has been removed in Locker 4.0+. Note that any files you already backed up will be automatically restored to the app when you open it. The app checks for any files that were stored in backup when it opens, but it is no longer possible to backup new files. We now recommend using your device's iCloud Backup feature to backup your entire device automatically.