Added Lane Sign
MUTCD W4-3
A lane is added ahead; merging is not required.
What it looks like
A yellow diamond warning sign with a black symbol showing an extra lane joining your roadway to indicate an added lane ahead.
When you'll see it
It appears where a new lane begins alongside yours, often at freeway entrances, so entering traffic has its own lane.
What to do
Continue in your lane; you do not need to merge, but stay aware of vehicles entering in the added lane.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.