HDR TV was the buzzword of 2019 in TV technologies. There are two main features which distinguish it from the average 4k TV.
Colors
While 4k provides a better viewing experience than 1080p, HDR makes use of the number of pixels to reveal more colors. The TV must be able to produce at least one billion colors to be able to make use of HDR content.
Contrast
The increased contrast between the brightest and darkest parts of a scene is what makes the images “pop.”
SDR, Standard dynamic range. TVs produce up to 500 nits, while HDR standard requires at least 1,000 nits for LCD/LED TVs and at least 540 for OLED. The latter has lower requirements because it compensates with deeper blacks.