That's notation used to refer to a technology called Chroma subsampling. 4:4:4 means it's not in use.
Chrome Subsampling is designed to reduce the colour resolution in a picture while keeping the shape resolution, because the eyes are more sensitive to detail in shape than in colour. It's a way to use less bandwidth/storage space without impacting the picture much.
Videos are generally stored as 4:2:0 so transmitting at full resolution doesn't make any difference there because the info isn't there to begin with.
The main impact is to small detail under certain colour combinations - text for PC use is often cited as an example.
If you're playing a colourful game that needs pixel-perfect precision (and you're sitting close enough to see details that small) then it might make a difference, but it's certainly not going to be a massive image quality improvement or anything.