AdMob

Ad mediation is a layer that asks several ad networks for a bid on the same impression and serves whichever pays most. Without it your app is tied to one network's demand and takes whatever that network offers; with it, networks compete per impression. Real-world uplift is commonly 15–40% higher eCPM versus a single network, and fill rate improves in smaller markets where any one network runs out of demand. The usual stacks are Google AdMob mediation, AppLovin MAX and Unity LevelPlay. Mediation is worth it once an app has meaningful daily volume — below a few thousand daily users, the extra SDK weight and setup cost outweigh the gain.

Above 95% is healthy for banners and interstitials; 70–90% is normal for rewarded video. Fill rate is filled ad requests divided by total ad requests. Low fill usually has one of four causes: a geography with thin advertiser demand, an eCPM floor set too high, a single network with no mediation behind it, or an app category advertisers avoid. Rewarded video legitimately fills lower than other formats because its inventory is scarcer — that is not a fault. If banner fill sits under 90%, add mediation before you touch anything else.