In plain language
Video is stored as brightness plus colour. 4:4:4 keeps full colour detail for every pixel. 4:2:2 halves the horizontal colour resolution. 4:2:0 halves it both horizontally and vertically, and needs roughly half the bandwidth of 4:4:4.
Why it matters in your setup
For film and television the difference is close to invisible — almost all broadcast, streaming and disc content is mastered in 4:2:0 anyway, so sending it as 4:4:4 gains nothing.
For a computer it is very visible. Fine coloured text on a 4:2:0 signal develops coloured fringes and looks smeared. That is why PC users chase 4:4:4 and why televisions often need a specific input setting before they accept it.
Subsampling is also the lever that makes demanding modes possible: 8K60 exists at 4:2:0 and not at 4:4:4, for exactly this reason.
What the DR HDMI 8K does with it
Its EDID tables let you advertise a specific chroma explicitly — 4K120 4:4:4 or 4K120 4:2:0, 4K60 4:4:4 at 600MHz or 4K60 4:2:0 at 12-bit. Rather than hoping the chain lands on 4:4:4, you tell the source which one is available. The DR HDMI 8K also reports the chroma actually in use on its OLED, so you can confirm what you got.
Need the hardware?
Open the DR HDMI 8K product page on HDfury.uk.