What does the blinking yellow arrow on the traffic light mean?

It means that you may make the indicated turn on a permissive basis, after yielding to conflicting vehicles and pedestrians.

This is usually the same as turning on the circular green, but the flashing yellow arrow can be displayed at times when the straight ahead movement can not be given a green light.

The purpose is to eliminate a dangerous condition called yellow trap. It occurs when the circular green is used to allow permissive turns, and one circular green ends sooner than the oncoming circular green. The left turning driver receiving the earlier yellow light tries to turn across traffic that still has a green light, causing an accident. The flashing yellow arrow can keep flashing until the oncoming green gets a yellow, allowing the driver to turn safely.