4:2:0 is the most aggressive of the everyday chroma subsampling modes — and also, by far, the most widely used. Every Netflix stream you watch, every YouTube video, every Blu-ray, every Zoom call, every modern broadcast signal in 1080p or 4K uses 4:2:0 to encode colour.
What it actually does: for every block of four pixels (a 2×2 square), the brightness of all four is kept, but only one colour sample is stored — shared across all four. The saving is dramatic — half the raw data of the video disappears — and human eyes cannot tell the difference on normal photographic or filmed content from a normal viewing distance.
Where 4:2:0 falls down is anything with hard, saturated colour edges next to each other: red text on a green background, a colourful UI in a screencast, certain animated graphics. Those produce visible coloured "fringes" along the edges. That's why professionals capturing screens for tutorials, broadcasters running on-air graphics, and colour-grading suites avoid 4:2:0 and stay on 4:2:2 or 4:4:4 — at the cost of file sizes typically 50–100 % larger.
