It's a type of lipid, a so-called sphingoid base that can be found naturally in the upper layer of the skin. It's found both in "free-form" and as part of famous skin lipids, ceramides.

There is emerging research about phytosphingosine that shows that it has antimicrobial and cell-communicating properties and is considered to be part of the skin's natural defence system.

A 2007 study showed that phytosphingosine works even against evil acne causing bacteria, Propionibacterium acnes and shows promise as a complementing active ingredient in treating acne-prone skin thanks to its anti-inflammatory and antimicrobial activities.  

A nice one to spot in the ingredient list. :)