Chroma subsampling is a clever shortcut that throws away part of the colour information in a video without you noticing. The human eye is dramatically better at seeing brightness changes than colour changes — you can pick out a tiny shadow or a thin black line instantly, but you'd struggle to tell apart two slightly different shades of green of similar brightness. Engineers exploit that by storing brightness (luma) at full resolution while storing colour (chroma) at lower resolution.
The notation 4:4:4, 4:2:2, 4:2:0 describes how aggressive the trick is. 4:4:4 is no subsampling — every pixel has its own full colour. 4:2:2 halves the horizontal colour resolution. 4:2:0 halves both horizontal and vertical colour resolution — only one colour sample for every four pixels. That sounds drastic, but tests with viewers consistently show they can't tell 4:2:0 from 4:4:4 on natural video at normal viewing distances.
The payoff is significant: 4:2:0 cuts the raw data of a video by half before any further compression even starts. Every consumer delivery format uses it — YouTube, Netflix, Blu-ray, broadcast TV, Zoom, Teams. The only places it isn't used are professional production (mastering, colour grading, VFX), screen capture (where text-on-coloured-background reveals chroma artefacts as colour fringes), and some medical and surveillance applications where every pixel matters.
