This is your proficiency with a small throwing weapons. It is automatically in effect while you are fighting.
This increases the damage done with small throwing weapons. Throwing weapons are inherently weaker than standard ranged weapons, as all the stats are in the ammo. This creates a dilemma: They can either make weak ammo that you can get a decent supply of, or strong ammo that's hard to get and/or will cost you a ton of money to keep using. As it is, the strongest small throwing weapon is WC60, whereas most weapons have a max of 80. There's also a small issue with the ammo having a size of it's own, especially since it reverts to it's previous size when you recover ammo from a corpse, and of course you can't refill ammo just anywhere, and thus you're limited by how much a specific store carries of that specific ammo per day. Aside from throwing cards, which illusionists can make. Still, if you don't have access to any other ranged weapons, it's not THAT bad. It's just that all other ranged options are better.