[2019] .ark files, what do they contain

As Ancient said, the timestamped ones are backups of your current save (Ragnarok.ark). You can revert to the backup save at any point by renaming one of the timestamped files to just 'Ragnarok.ark[\b]. Make sure you keep the original Ragnarok.ark file though in case you need to go back to it if something is wrong with your backup save(s).