Weapon modifiers are items found on the map that can be placed on a weapon to give it an additional buff / stat. Modifiers share a similar appearance to upgrade kits in the inventory, but are only related in that both gives stats/buffs. When picked up, it shows a icon as it goes into your bag.
Once a modifier is placed on a weapon, it can not be removed or replaced. If of average+ quality, they can be salvaged for 2 1 .