Irish Moss is a seaweed derived fining agent used by many brewers to help make a clear beer without the need for a filter, and to prevent chill haze. Irish Moss accelerates protein coagulation during the end of the boil which helps prevent chill haze. Many brewers use this product in every batch. 

Common usage or per instructions: 
Use 1 teaspoon per 5-gal(19L) batch, add with 15 minutes remaining in the boil.