/var/backups/md

You should have raid_config_XXXX_XX_XX_XXXXXX.tar.xz files in /var/backups/md

Has support looked at your system for some reason? Sounds like they extracted one of those files but forgot to clean up afterwards.

The compressed archive can be invaluable in a data recovery situation. So leave the tar.xz files but you can delete everything else in /var/backups/md

The files in /var/cores are core dumps. These can be useful for diagnosing why those services crashed. If they're recent you may wish to compress them e.g.

# cd /var/cores
# lzma -9 core-readynasd

and keep them around in case they're needed for troubleshooting or if they're very old you could just remove them.

/var/lib/apt/lists contains the list of available packages to install.

/var/cache/apt contains the package archive of packages that have been downloaded.

/var/log/journal contains the main log for the system (this is rotated automatically).

/var/readynasd and /var/backups/readynasd contain important databases and backups respectively. Leave these alone.