What is running before the wind in sailing?

"Running before the wind" is when your craft is sailing directly downwind, or "with the wind".

For greater speed caused by less drag in the water, you may raise your center- or dagger-board, as steerage is not critical. If you have a jibsail, you may set it out oppositethe mainsail to "run wing and wing". This is the slowest form of sailing the existing winds.

Care should be exercised in light wind conditions, as even the slightest swells will cause the mast to sway forward and back, possibly luffing your sails.