Suspended protein molecules cause a lack of clarity or cloudiness in beer, this is known as a "protein haze". 

To combat protein haze, brewers add Irish Moss to the boiler when making all-grain (mash) or extract beers. Irish Moss is a sea weed extract the function of which is to encourage protein molecules to aggregate into larger particles so that they will settle out of solution.

Dosage: Add one teaspoon per 20L of wort, 10 - 20 minutes before the end of the boil.