In plain English
Everyone on this road is going the same way. That changes things: you can pass on either side, and you should pick your lane early for where you're going.
Where you'll see it
Town centres and gyratory systems, usually paired with no-entry signs at the other end.
The common trap
Confusing it with "ahead only" — a blue circle. The circle orders your move at a junction; the rectangle describes the whole street.
Memory anchor
A rectangle is a road's name badge: "Hi, I'm one-way." A circle would be a command.