Where:
On any road with more than one traffic lane in the direction you are travelling.

Purpose:
To warn that the multi-lane road you are travelling on is about to reduce in width, and one of the lanes (either on the left or right) will end.
This means the number of lanes available for traffic in your direction will decrease by one.

Action:
Observe the sign carefully to see which side the lane is ending on.
Check your mirrors and blind spots, signal your intention early, and merge safely and smoothly into the lane that continues.
Do not wait until the last moment to change lanes, as this can cause congestion or collisions. Yield to vehicles already in the lane you are moving into, and adjust your speed to maintain a safe following distance.