At a rough guess I would say it means that you have no accounts that have had a debit balance that you have then fully paid to bring back to a zero balance.
In other words that you have held a credit facility and have run up a balance of some kind, say a credit card or a loan (not a mail order account such as a catalogue or Ebay or Amazon) and have then reduced this balance to zero again through repayment of the debt. The fact that you can demonstrate an ability to have finance and then successfully repay the facility that you have arranged will give you credit scoring "brownie points" and this is something lenders like and therefore boosts your score
I don't think that this is altogether a negative thing, it is just something that is holding your score back from being a potentially higher score. In other words, you may have a score of 800 right now but if you had previously settled a non mail order account then you may have had a score of 999 just for having settled the account.
I doubt very much that this means that it is reducing your score to a lower level than it is, only reducing the potential maximum score that it could be.
If that makes sense.......