From the original documentation:

Symbiosis is a developer tool for adapting Mac OS X VST plug-ins to the Audio Unit (AU) standard. It essentially consists of a single C++ file that you can compile into your VST project to make the plug-in compatible with the Audio Unit protocol. Optionally, you may choose to use a pre-built "wrapper plug-in" for adapting your existing VST plug-in to the AU standard without even having to recompile any source code.

Symbiosis was developed and open-sourced by Magnus Lidström of Sonic Charge fame. It was originally hosted on Google Code but doesn't appear to have been actively maintained for some time now (the last commit by Magnus was on April 22, 2013). With the closure of Google Code, the question was asked on the Symbiosis mailing list / group whether the official repository would move, but there was no response from Magnus. In the meantime, several people have imported the Symbiosis Google Code repository into GitHub.

This repository is also an import of the Symbiosis repository. It contains some updates that are useful for the NUsofting plugins: the ones on the develop branch are those that seem to be more generally applicable, the ones on the NUsofting/develop branch are increasingly large divergences from the original Symbiosis where we no longer need the full flexibility that it originally provided. This is especially the case where we can provide a more direct interface to our plugin framework, and don't want / need to be limited by the VST 2.4 interface.

This version of the project also contains updates to support the Audio Unit version 2 API.