To learn to use the CLI, you can read the wiki:
http://forum.synology.com/wiki/index.ph ... ng_the_CLIBasically, everything what you see after you type "cd /" is system partition, except the directory volume1, volume2, etc. which are mountpoints to data partitions.
I suggest that you start with "du -hs /var". /var is supposed to contain temporary (VARiable) data.
If the number that you get is not that big, scan all the directories with:
"du -hs /[a-u]*". This command will give you the size for each directory beginning with a letter from a to u.
Anyway, yes, 238MB is not much left, and IMHO it can cause problems. As you should have started with DSM 3.0/3.1, you should have at least 1GB of free disk for /dev/md0