Beating is zig zagging up toward the wind.  Pull in your sheet so the sail is in the close haul position.  Do not not let the sail luff, and if it starts to luff, you should bear off or come about unless that would cause you to foul another boat. 

sheet - the rope that pulls in the sail

close haul - when the sail is pulled all the way in

luff - flap

bear off - head away from the wind

come about - turn when going toward the wind