Edge Version 89.0.752.0 (Official build) canary (64-bit)
The button doesn't have a tooltip yet but what is does is basically enabling/disabling writing on the screen on touch-based displays.
it has no effect if you are using mouse and keyboard, but when you use Edge with a touch screen display, you can only write on the web capture or PDF while that button is toggled, if it's off, you can't write/draw anything.
as always, it might only be available to a few insiders.