I'm a VSS user learning how to use SVN, and I have selected TortoiseSVN as my platform. Can someone tell me what the term "HEAD Revision" means?
It means the Latest Revision.
Getting the HEAD Revision from SVN would be like doing a 'Get Latest...' in VSS.
The latest revision that you committed to the repository.
Example: If your last commit created revision 15, then 15 is the HEAD revision
The Tortoise help file is well worth a read. From the Glossary:
HEAD revision
The latest revision of a file or folder in the repository.
Since your question is Subversion-specific, here is what the SVN book says:
HEAD
The latest (or “youngest”) revision in the repository.
For comparison, here are the other revision keywords:
BASE
The revision number of an item in a working copy. If the item has been locally modified, this refers to the way the item appears without those local modifications.
COMMITTED
The most recent revision prior to, or equal to, BASE, in which an item changed.
PREV
The revision immediately before the last revision in which an item changed. Technically, > this boils down to COMMITTED−1.