Short Answer: Pixels. Long Answer: A screen's resolution refers to how many pixels it can have. A TV with 4K resolution has twice the pixels as one with 1080p HD. Having more pixels means you can have better detailing and color ranges, and even better contrast when the pixels are individually lit like in OLED screens or TVs with contrast zones. If you want to read more, you can check out our article that explains 4K resolution in-depth.