There's a lot of lingo around the wider web so this is one of a series of short definition posts.
Overlay controls on the wider web float over the real or virtual environment that you see in portal displays. They can be two dimensional like a button or they can be three dimensional and react to the environment.
For example, you might have an overlay control that seems to float on the surface of your mobile phone in portal display mode but changes color when it is over a real item like a chair or a boat.
Overlay controls are one of the three types of controls on the wider web. In the "Overlays Controls" section of the image below, the overlay controls are the title, "PotassiumES", the "Home | About" toggle switch, and the "Exit AR" button:
Using Three.js to build a reusable border geometry for spatial UIs!
🌸 Updated potassium style system (KSS), now with margins!
🌸 The path to the vNext
🌸 A couple of new spatial controls
🌸 A brief intro to the existing samples
🌸 Building UI components that work in flat, portal, and immersive display modes