That depends a bit on your interpretation... It could either mean it increases the changes of improving the Barrows items (which are more impressive than normal drops) or it increases the chances of gaining a better barrow item if you would get one anyway. However, I believe that Barrows items are equal in the game (players may value one higher than the other, in the game they're the same level) so I believe it simply increases your chances of getting any Barrows item.