Pooling is the step that collapses a full-reference metric's many per-frame scores into the single number you report for a clip. A ten-second clip at 30 frames per second produces 300 scores, and pooling combines them the way a teacher turns many marked assignments into one course grade. The catch is that the per-frame scores are fixed but the headline number depends entirely on the rule you choose, so the reported score can move without a single pixel changing. The default in nearly every tool is the arithmetic mean, which weights every frame equally and therefore hides a short stretch of bad quality inside a long good one. Better choices surface the worst frames: the harmonic mean tilts toward low values, percentile pooling reports the worst 5 or 10 percent, and the minimum reports the single worst frame. Pooling happens twice, once across pixels inside a frame (spatial) and once across frames over time (temporal). Always state which pooling method you used.
