From michigan.gov
RULES OF THE ROAD Question: I recently encountered a new traffic light with 4 different signals including flashing arrows in red, yellow, and green. What am I supposed to do when the light is flashing a yellow arrow?
Answer: The Michigan Department of Transportation [[MDOT) has recently begun replacing the old flashing red signals for left turn lanes at intersections with a new style of signal that incorporates four lights. This is what MDOT has to say about the new signals. "Those lights are a flashing yellow arrow which permits a left turn when oncoming traffic is clear [[oncoming traffic has a green light), a steady green arrow allows you to turn left, a steady yellow arrow warns that the left-turn signal is about to turn red and you should prepare to stop, and a steady red arrow which requires you to stop. The steady red arrow will be followed by a flashing yellow arrow on the next cycle."
For more information read this brochure on the New Four-Arrow Signal.