Fill rate is the percentage of ad opportunities (impressions requested) that the ad stack manages to fill with a real, paying ad. An unfilled slot is lost revenue - and a bad viewer experience if it leaves a gap or shows a slate - so fill rate is a key health metric of the monetization stack.
It depends on demand: how many advertisers want this audience, how well the platform is connected to demand sources (direct deals plus programmatic exchanges), and floor pricing. There is a constant tension between fill rate and CPM - lowering the price floor fills more slots but at lower value - so optimizing ad revenue means balancing the two, often with fallback waterfalls or header-bidding-style auctions.
