A two-way left-turn lane is very common.
The sign tells you that the entire lane allows you to set up to turn left.
Once you enter the lane you must turn left at the first exit.
It is a good place to set up because it moves you out of the traffic flow into an empty lane, a safe place to wait.
Think of it as an unusual waiting position for a left turn.
Entering too early can cause you to be in conflict with a head-on vehicle that is also entering to turn their left.
Occasionally you may see two opposing cars enter too early. Both just stop and wait for one to complete their left.
Once in this lane, you should not move back out of it.