A feint is a 'fake' attack meant to mislead your opponent and make them more vulnerable to followup attacks. This skill allows you to use feints in battle.
If you successfully feint, your enemy will be open to an attack of opportunity, and will have a harder time dodging or parrying blows.
Enemies with a high dexterity compared to yours will be harder to feint, and enemies that are smarter than you are will also be harder to feint. Note that truly stupid, unintelligent entities are completely unaffected by feints, as they lack the intelligence to plan ahead.
Also, if your enemy is fighting multiple targets, it may not notice your feint, essentially acting as a failure.