Chroma subsampling
Storing colour information at lower resolution than brightness — the eye barely notices, and it halves the data.
Human vision has far more receptors for brightness than for colour. Almost all video exploits this by splitting the image into luma and two chrominance channels, then storing the colour more sparsely. The notation 4:2:0 means colour is sampled at half resolution both horizontally and vertically.
The saving is dramatic and the cost invisible — almost. Sharp colour edges go soft, which shows most clearly on red text against black. It is also why a computer connected over HDMI sometimes looks blurry in text: the display has fallen back to 4:2:0 rather than 4:4:4.