It's a C++ pub-sub framework (a lot like ROS, in that way), but rather than declaring C++ publishers/subscribers directly, you declare the topics and types your code cares about in a configuration file, along with conditions on those inputs. Doing this allows for deep knowledge about the code running, which unlocks the possibility of deterministic simulation and testing (along with making it easy to generate bindings for other languages, create alternate schedulers, more easily swap out internal concepts, etc). It also enables easy static analysis of the codebase - given a launch file and arguments to the launch file, one can analyze the topic network and find missing publishers or publishers that publish to nothing.