Assuming this is the electric (aircon) fan, it means an overheat condition has been detected. I believe that this can latch so that the fan always comes on but when mine did it the fan went off when everything cooled down and coolant replenished.
You really need to get it checked out with nanocom / hawkeye as the ordinary temperature gauge stays on the mid position even when the engine is getting well above its normal temperature.
On the other hand, it may just be the aircon working properly......
