In British Columbia and Massachusetts, a flashing green means that the cross street or crosswalk changes on demand, and your flashing green right of way may turn into a red light at any time. It doesn't differ much from a flashing yellow, I guess, except that it may change to a red.
In Québec, a flashing green means you now have a protected left turn. It is equivalent to a green left arrow.
In the former Soviet Union, it means neither of these things. What it appears to mean is "Look out, your green light is about to turn yellow", and it follows a steady green light.
Not all traffic lights have flashing green lights, so it's difficult to be sure, but I can tell you from watching the lights, however, that it does *not* mean what it does in Québec, and if you treated it as one, you would get broadsided by someone accelerating to beat the yellow.
You wouldn't think people could come up with this much variation in a simple traffic light, would you?