In plain English
The road is about to curve left harder than it looks. Slow down before the bend, not in the middle of it — braking mid-corner unsettles the car.
Where you'll see it
Country lanes and slip roads, usually before bends you can't see around.
The common trap
Confusing the bend warning (one curved arrow) with a chevron board — chevrons mark the bend itself; this triangle warns you before it arrives.
Memory anchor
The arrow draws the road's path for you. Whatever the arrow does, your car is about to do.