In plain English
You can drive in, but you can't get out the other side. The red bar across the top is the wall; the white stem is the road hitting it.
Where you'll see it
At the entrance to cul-de-sacs and residential closes.
The common trap
Mixing it up with no entry. No entry (red circle, white bar) bans you from entering at all. This one lets you in — it just warns the road goes nowhere.
Memory anchor
A T like a dead-end wall: the white road runs straight into the red bar and stops.