TortoiseSVN: What is head revision

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.

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.

It is the very latest/current revision of the project.

Wikipedia defines it as:

Head The most recent commit.

See Common_vocabulary.