In plain English
Not slow down — stop. Wheels still, even if the road looks empty. It's used where the view is so poor that only a full stop gives you time to look properly.
Where you'll see it
At junctions with terrible visibility, where creeping out without stopping has caused crashes.
The common trap
Doing a rolling "almost stop" when it looks clear. If the junction were safe to roll through, it would have a give way sign instead.
Memory anchor
The only octagon on the road. Eight sides, so you know it's STOP even from behind or covered in snow.
Quick check