The foamy head that forms when primary fermentation is most active.  It is imperative that you give your wort enough headspace so that you don’t have a messy overflow, or even worse, a blowout.  I once shot the top off of my air lock due to limited headspace.  Enough pressure formed inside the carboy and skyrocketed the cap off and yeast/wort got all over the walls of my brew room.  The krausen is usually a creamy color and consists of the yeast and protein carbon-dioxide filled bubbles and usually settles out after 3-4 days.  It usually leaves remnants behind, among the walls of the fermentor.  Krausen is normal, and as gross looking as it is, it represents a healthy and active fermentation.

Krausen forming during active fermentation

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