Athena is the name of the ATLAS software framework that manages almost all ATLAS production workflows: event generation, simulation, reconstruction and derivation production. Athena is also used online in the ATLAS High Level Trigger.
This guide gives some useful practical information about working with the framework and how to develop new code.
Nota Bene this guide covers development, builds and deployment for modern Athena releases (21 and later) where the code is managed in git and built using CMake.