PQ stands for Perceptual Quantizer. It's the technical recipe — formally called SMPTE ST 2084 — that converts pixel numbers into real-world brightness for HDR video. Without PQ, the same code value would mean different things on different screens; with PQ, the standard nails the brightness scale precisely from 0 nits (pure black) up to 10,000 nits (extreme highlights, brighter than anything any consumer TV can yet display).
What makes PQ clever is that it's tuned to how the human eye actually responds to light. Our vision is enormously more sensitive to small brightness differences in dark scenes than in bright ones, so PQ doesn't space its code values evenly — it allocates many more levels to the dark range and far fewer to the brightest range. The result: even in 10-bit (1024 levels), there's enough resolution to show a starlit night and a noon-sky cloud in the same scene without banding.
PQ is the foundation of HDR10, HDR10+ and Dolby Vision — all three use the PQ curve as their brightness language. Its main limitation is that it's absolute: the encoder specifies "this pixel is exactly 800 nits", and if the viewer's TV can only reach 600, the player has to do tone-mapping to gracefully squash the highlights. This is why HDR10 looks fine on most displays — players do the brightness translation behind the scenes. The alternative for HDR, HLG, takes a different approach and is preferred for live broadcast.
