This skill lets you do more damage with long blades and parry with them more reliably. Note that no level of mastery will let you do great damage with mundane weapons.
The ingame help reads:
Skill duration: random. Type of skill : weapon skill. Affecting stats: Str/Dex. It uses no endurance points. One of the favorite weapons over the years, is the sword. Varying in size and shape, they come in many forms. While anyone who picks up a sword for the first time may be able to swing it back and forth, it takes much practice in order to wield a blade with true skill. Much of the art called swordsmanship is made up of discipline. Executing the same movements smoothly, and fluidly over and over again are what give a real blademaster his edge. A true master can take his sword, and swing with only a bit of effort, and slice his foe in half, while a beginner may swing with all his might, and not even penetrate his foes outer skin.