In plain English
Blue circles are positive orders — this one says the only legal move at the next junction is a left turn. Plan your lane and signal early.
Where you'll see it
At junctions where going straight or right would send you into a one-way street or banned route.
The common trap
Mixing up blue circles (you must) with blue rectangles (just information). Shape first, colour second.
Memory anchor
Blue circle = "do". The arrow isn't a suggestion — it's your only option.