15^50 is a descriptive slang term applied to a martial weapon with the "Damage +15% while Health is above 50%" modifier, either as an inherent mod or from a perfect "Strength and Honor" inscription. These weapons tend to be more highly sought-after than those with other inherent modifiers.
These weapons are usually found as high quality rare drops. In addition, some special items always have this modifier, such as the Icy Dragon Sword, some Collector weapons, and many green items.