What are Overlays?
Overlays are images, text, alerts, etc that show up within your video player. These often are added within your streaming software using a third party service. A quick internet search on "how to add a stream overlay" should provide many tutorials depending on your streaming software of choice.
How do I use overlays with Xbox One or Xbox Game bar?
Do you stream from Xbox One or Windows 10 game bar and want to setup an overlay? Please see here on how to do so with Lightstream.