This sign is posted on a multi-lane road ahead of you. What does it mean?
One of the lanes ends, and traffic in it must move into the lane that continues
The sign is about the lane rather than the road: the pavement ahead has one lane fewer than it does here. Reading it early lets you either move over or leave room for a driver who has to, which is what keeps the pinch from turning into a scramble at the taper.
Highway Traffic Act (Ontario)