Gybing is turning (tacking) the boat with the wind behind you.  First you lower the daggerboard to make it easier to steer the boat.  Then  you pull in the sheet in so the wind can more easily get in front of the sail.  Then you bear off to leeward.  Lastly, yank the sheet to start the gybe.  Duck you head as the boom passes over you.  Move your body to the other side, and lower the daggerboard.

daggerboard - the big white plank which slide into the hull

sheet - the rope which pulls in the sail

bear off - to steer your boat away from the wind

leeward - the side away from where the wind is coming from

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