Chroma subsampling

Human vision reads brightness far more precisely than colour, so codecs discard colour first. The three numbers describe the sampling pattern: 4:4:4 keeps full colour for every pixel, 4:2:2 halves it horizontally, and 4:2:0 halves it in both directions and is what almost all delivered video uses.

The difference shows up when you push the picture. Green screen keying, heavy grading and small coloured text against a contrasting background all fall apart faster in 4:2:0. That is the argument for recording 4:2:2 or better even though the final file will be 4:2:0, because the extra colour is there while you work rather than in what the viewer receives.