Lumen is the result of years of research at Epic to bring real-time Global Illumination and reflections to Unreal Engine. In this video, we’ll show what it is, talk about its features, and walk you through a high-level overview of how it works.
https://dev.epicgames.com/community/learning/talks-and-demos/Ede/lumen-in-ue5-let-there-be-light-unreal-engine