Addons are attached services to your main instance. Typical examples are: caching systems, databases, etc.
Addons can be added or removed via the administration, under Settings. Once created, they are deployed automatically when you deploy your instance. You can access them using their name (hostname).
Addons definitions are fully open source and specifications are available in this repository: https://github.com/openode-io/addons Users can contribute, add new addons as well as change existing ones.