OpenEmbedded is the build system used by AsteroidOS, it allows to easily maintain embedded linux distributions. OE is also used by project such as WebOS-Ports, SHR or Ångström. OpenEmbedded is supported by the Yocto project from the Linux Foundation and provides a rock-solid basis for AsteroidOS.
OpenEmbedded uses the bitbake tool which is developed in python to generate images (like asteroid-image) from packages (like asteroid-launcher-dev) built from recipes (ex: asteroid-launcher_git.bb) provided in layers (ex: meta-asteroid).
Layers are git repositories containing a bunch of related recipes, example of layers include board support packages like meta-dory-hybris or meta-rockchip which describe a way to support a new machine, but also UI layers like meta-xfce or meta-gnome which describe the building process of graphic components.
A recipe describe how to fetch, patch, configure, compile, install, package (in .rpm, .deb or .ipk) and test a piece of software. Bitbake handles all those operations and dependencies between them and between recipes. In the end it can generate images or SDK for multiple targets.
Contributing to the AsteroidOS’s OE architecture mostly consists in maintaining the meta-asteroid and meta-*-hybris repositories which contain the recipes related to Asteroid and the watches BSPs.
In depth info can be found in the Yocto Project's manual.