Rec. 709 refers to the International Telecommunication Union’s standardization of the high definition television format. It includes standards for pixel count, frame rate, aspect ratio and color space, so that content providers can format footage for the optimal viewing on a television.
Specifically for the standard color space, cinematographers use “Rec. 709” as short hand to refer specifically to color graded footage that conform to the standardized color space. Though it is a major step above standard definition color space, Rec 709 only encompasses about 36% of the entire visual color spectrum. Simply applying this grade may desaturate and alter the desired color composition of a shot.