Dynamic range: the spread between the darkest and the brightest a TV can be. In other words, contrast ratio. But having beaten that term into the ground with a decade of dubious marketing use, it was time for new terminology.
Except, this isn't just marketing fluff. High dynamic range, in its multiple forms, can be a significant and noticeable increase in picture quality. Multiple companies are talking about it, from Dolby and its Vision tech, to Sony, Samsung, and others.