Fill rate is the share of an app's ad requests that come back filled with an actual paying ad. When the app asks the [[ad-server]] or [[ad-mediation]] layer for an ad, the request is either filled (an ad is returned and shown) or unfilled (no ad, no revenue). Fill rate is the ratio of the former to the total.
A low fill rate is lost opportunity: the user reached a moment where an ad could have shown, but nothing monetized it. Causes include thin demand for the geo or format, a [[floor-price]] set above what buyers will pay, or an [[ad-network]] with limited inventory for that audience. The usual fixes add demand — more networks in the [[ad-mediation]] waterfall, or [[header-bidding]] to auction every impression across sources at once.
The catch is the floor-price trade-off. Raising your floor lifts the average [[ecpm]] of the impressions that do fill, but it also rejects more bids, so fill rate falls. Push floors too high and you earn a great price on a small slice of inventory while the rest goes unfilled. Because of this, fill rate is never optimized alone — what matters is net [[ad-revenue]], roughly requests × fill rate × eCPM, so fill and price are tuned together.