HDR, if you're unfamiliar with the term, basically increases the "range" of colours and brightness in your TV picture. It doesn't rely on extra pixels, but instead highlights darker parts of the screen while simultaneously boosting the brighter areas. Fundamentally, it means you'll see greater detail in low-light areas and colours will feel a lot more realistic. Samsung already sells a number of HDR-supporting sets, which you'll need to have in order to watch Amazon's updated streams.
Talking of streams, the company has only made the complete first season of Mozart in the Jungle and the pilot of its upcoming Red Oaks show available in the superior picture quality, but more will come to Prime in the near future. In that time, we can expect Netflix to also get in on the act, giving subscribers more choice over how they watch their favourite shows.