Baloo and extended attributes
Applications such as Dolphin store tags and ratings in xattrs (extended attributes) in the filesystem, and Baloo indexes these so you can search them. On Linux, to show them as entered in Dolphin, in a terminal enter:
(You may need to install the package that provides
getattr. On Debian, run
sudo apt install xattr in a terminal).
getfattr --absolute-names -dR /home 2> /dev/null # recursive search under /home/
To open e.g. textfiles found in this manner, manually add a webshortcut
and name the shortcut 000ffoxopener to conveniently open the found and highlighted file after right-clicking... shortcut... with e.g. firefox right from within Konsole.
Some filesystems (e.g. FAT32, the filesystem with which most USB flash drives are formatted) do not support xattrs at all, so copying files to such a volume will lose the tags.
Most CLI utilities (e.g. cp, tar, rsync) won't copy xattrs without additional flags either, so unless your environment is 'All KDE all the time' and you only use Linux filesystems like ext4, XFS, or Btrfs (this is not a complete list), and you modify your backups to specifically include xattrs, your tags may get trashed one way or another.
Nepomuk (the previous KDE tagging/file metadata system) was used to store tags in a separate database, but even after many years of development, it could hardly be made to work performantly or reliably so it was replaced with Baloo.
In some ways using xattrs is 'the right place' to put this tagging info, but Linux doesn't have a 'core API' that would allow a 'xattrs preserved by default, and interpreted consistently' policy. Therefore there are a lot of ways to lose your tags. KDE has been trying to make this work for many years, but it is difficult given the way UNIX/POSIX has evolved.