Using the Upgrade tab at the forge you can unlock weapon traits. The number of traits a weapon can have is based on it's color/quality. Uncommon weapons (green quality) have 1 Trait, Rare weapons (blue quality) have 2 Traits, Exotic (orange quality) and Veteran (red quality) weapons have 3 Traits. Upgrade tokens are consumed to unlock each trait. The type of token used to unlock a Trait is one level below the weapon itself (i.e ]. green weapon traits are unlocked with white tokens, blue weapon traits are unlocked with green tokens, etc.) The cost of unlocking goes up for each successive trait that is unlocked on the same weapon.
After the first time you Upgrade and item this Trophy will pop.