When you find a weapon, you can reroll the stats of it. I found this term quite weird since there isn't really a reroll being performed it is more of an upgrade. Either way, you need to upgrade different weapons 100 different times. You will likely get this as time goes on. If you need to grind it, go for the lower tier weapons. It is cheaper to reroll the low quality weapons.
Reroll weapon or equipment attributes 100 times