In short, it means Super Soft Wool. Mercerizing smooths out wool fibers – leaving all the great natural properties of wool, with none of the itch! When magnified, natural wool has what are referred to as "scales". Think of it as looking like barbed-wire... That's why it's so itchy. The mercerizing process is the equivalent of putting the wool fibers in a rock tumbler, smoothing them out & making it more appealing (in this case, more wearable). This process makes the wool Softer, Stronger, Lighter, More Supple, and Anti-Pill, but keeps the natural benefits of wool – Anti-Microbial, Odor Resistant, Moisture Wicking, Quick Drying, Breathable, etc...
Say goodbye to shirts that are itchy, shrunken, and pilling.