If someone chooses to downvote something, it gets less rewards. The more stake downvoting it (whether from one person or multiple), the more stake disagreement there is, and the less rewards it gets.

At the same time, OTHER posts which do NOT get downvotes (or get relatively little stake downvoting, even if some), get more rewards. Those are the ones where the community has broadly agreed to direct rewards.

You can think of a downvote as a sort of veto on payouts. Anyone (or multiple people) can stand up and object, essentially veto that payout, though strength of that veto depends on stake.