A boat is running when the wind is coming from behind it.  Generally a Sunfish sails fastest when it is flat side to side and bow to stern.  However, on a run if the boat is level side to side, it tends to turn into the wind, known as a weather helm,  unless it is heeled to windward.

bow - the front of the boat

stern - the back of the boat

weather helm - boat tries to head into the wind

heeled - the boat tips to one side

windward - the side the wind is coming from