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.
Once a modifier is placed on a weapon, it can not be removed or replaced. Trying to do so will just swap the locations of both items like normal items do when dragged onto each other.
Before V1.30 weapon mods didn't have a specific icon; instead they showed the icon of the related buff just like upgrade kits did. This was problematic however as it was hard to tell at a glance if an item was an upgrade kit or weapon mod, and some players would even scam others when trading. Since it had no icon, when picked up it used to show a wire icon as it goes into your bag.
A strategy to make use of weapon mods permanently is to put them on guns (preferably Dirty Harry's gun, which can't be accidentally salvaged) and switch to them quickly when needed. For examples, a gun with EQ Experience 20% can be equipped before picking experience in bags and a gun with EQ Regeneration 50% can be swapped in outside of combat to gain health back faster.