An accessibility overlay (or accessibility widget) is one of those wee icon-only buttons you sometimes see floating over a web page; usually in the bottom-right corner. When you press them, they give you a bunch of options to make the website ‘more accessible’.
Why did I use inverted commas? Well, they profess to make a website more accessible, but do little-to-nothing of any real use; in fact, they can actually hinder a disabled user’s experience of a website!
https://www.tempertemper.net/blog/accessibility-overlays-are-not-for-disabled-people