nymea is an open source IoT stack. Its main components are the core, the plugins and the client app. Do you want to enable your hardware to work with nymea? Plugin development is what you want. Do you want to create your user interface? Check out nymea:app as a reference implementation. Eager to extend the overall platform? Dive into core development!
The nymea core is main part of the whole stack and does the overall management. Check the docs for a detailed description on how it works and how to speak with it.