The overlay is the on-stream ad server that renders a booked ad over a creator's stream through one OBS browser source; it is what makes measured exposure possible, because the ad and the stream become the same picture.
The on-stream ad server: the layer that renders a booked ad over a creator's stream through one browser source in OBS. It is the piece that makes measured exposure possible, because the ad and the stream become the same picture. See the overlay.