In the forums you will see references to Drupal HEAD or just HEAD.
You will also see references to Drupal CVS which is often the same
thing. The name comes from the fact that Drupal uses the CVS
version control system to track changes to the code, and in CVS, "HEAD"
refers to the main development area (also known as "the trunk").

HEAD is the name given to the version of Drupal core being worked on
by developers right now. However, now that core is only using two
digits for the version number (starting with the 5.0 release), there's
no longer any ambiguity about what the next version of core will be
called, and the use of "HEAD" to identify a release is no longer
necessary. For example, as soon as the official 5.0 release of Drupal
core is out, everyone will know that the next version of core under
development will eventually become the 6.x release series, so the
nightly snapshot releases will be called "6.x-dev", not "HEAD". It is a
moving target, and prone to serious bugs and even security holes. The HEAD is not meant for use by regular users on production sites and should only be downloaded and used by developers very familiar with Drupal or those wishing to help find bugs in the software.

Note however that a module, theme or other file tagged with HEAD
only means that it is the latest file. But latest can be anything,
sometimes over a year old! When a file was made for Drupal 4.3.x, it is
HEAD, but it does not mean you can run it on 4.7.x, or some other HEAD.

With the new release system, there is no good reason anyone should
have to use a HEAD release of any contribution. Furthermore,
contributions added after the new system was put into place will
require that even their HEAD release will have version information and
indicate what version of Drupal core the release is compatible with.
Therefore, in general, production sites should never use HEAD releases,
and should always use official releases with real version numbers if
available. A contribution without any official releases is an
indication of a contribution maintainer not being very responsible, so
you should use these contributions at your own risk.