HDR is known as High Dynamic Range. HDR creates more contrast between the highest and darkest areas in an image. HDR affects brightness and contrast ratios.

Nits are a measurement of a unit of brightness. It describes how bright a display device such as a TV, monitor, or smartphone is. It measures the luminous intensity (the candela) and square meter. "candela per square meter."  (ex. HDR400 = 400 nits.)