This directory contains downloadable versions of Heron and some Heron-apps, like the PreviewApp.
Under "older_releases" are older releases of Heron for the sake of history.
See the Heron roadmap for details on what is in each release:
https://code.google.com/p/geoext-viewer/wiki/Roadmap

STABLE
Download the file "heron-<version>.zip where version should be something like "1.0.1".

UNSTABLE
heron-unstable.zip is the current SVN master branch "trunk" version packaged as Heron release
Use this only if you have specific requirements or are testing the latest developments.