Coming about is turning when the wind is in front of you.  You start on one tack - either starboard or port - and end on the other tack.  When you come about, you change the hands that hold the tiller and sheet, move your body to the other side, and duck you head so the boom doesn’t hit you.

tack - a direction

starboard - the right side of the boat

port - the left side of the baot

tiller - the stick used to steer the boat

sheet- the rope used to pull in the sail

boom - the metal or wood rod at the bottom of the sail