What does it mean if a cover "breathes"?

Boat Covers Direct offers only breathable materials for the benefit of your boat and it’s interior. Being breathable means the cover is porous allowing air to pass through letting any condensations or moisture evaporate out, helping to prevent mold and mildew issues. But don’t worry, even though the material is breathable, it will still repel water. Carver covers are treated with the latest technology in water-repellency to keep your boat protected and healthy.