Show rate (or render rate) measures delivery after the auction. An ad can be won and cached, then never shown because the session ended, the placement was skipped, or the creative failed to load.
It matters most when comparing demand sources or mediation setups: two bidders with identical eCPMs can contribute very differently if one renders reliably and the other does not. Latency and cache strategy both influence it.
Track it per placement. A healthy blended show rate can hide one important placement that is quietly failing.