I'm guessing that it's indicating that Data Saving is on. If you go to Settings > Connections > Data usage > Data Saver, I think it will be switched on. Mine by default is off. With Data Saver enabled, your Android handset will restrict the background use of cellular data, thereby saving you from any unpleasant surprises on your monthly mobile bill. I'm on an unlimited plan so there is no need to have the saver option enabled.
If that isn't the icon you are seeing... I have no clue.