Not sure what your experience is with mysql, so I apologize if this is too detailed. (But it will help others in the future, so it's all good. ) I'll assume you at least know how to get into mysql in the Zimbra CLI.
Each mailbox group has its own set of tags. Here's what I did to see the tags for my group, 40.
(If you know SQL, you know some of these steps are not necessary, but they are instructive for people who aren't familiar with databases.)
> show databases;
> use mboxgroup40;
> show tables;
> show columns from tag;
> select * from tag;
I just realized you may have already created the tags themselves, and are wanting to know where they are kept for individual mail items. If so, that's in the tagged_item table in the mboxgroupxx database.