It's a job scheduler that runs directed acyclic graphs. A key feature of Airflow is you can more or less write your workflow naturally in python, and it will create the workflow graph for you. It includes a web interface for managing your workflows, a scheduler, and much more.
Here here! The docs don't really make it clear what Airflow is for someone who hasn't already used or been exposed to it. Is it a job scheduler like Rundeck? Something else?