In plain English
Traffic on the road you're joining has priority. Slow right down, look properly, and only roll on if it's clear. You don't have to stop — unless you need to.
Where you'll see it
At most junctions where a minor road meets a bigger one, usually with a broken white line painted across your lane.
The common trap
Forgetting it's the only point-down triangle on UK roads — examiners love asking why. It's so you can recognise it by shape alone, even snow-covered.
Memory anchor
The upside-down odd one out. Point-down = polite bow: "after you."
Quick check