Editing & Post-Production
Rec. 709
Also called: Rec709, BT.709, standard dynamic range
Rec. 709 defines the primaries, white point and transfer curve for standard dynamic range HD. In practice it is the target you grade towards, the thing a monitor gets calibrated to, and the space a viewer's screen assumes when nothing else is signalled. Footage shot in a log profile looks flat and washed out because it is holding extra range, and converting it to Rec. 709 is what makes it look like a normal picture again.
Uploading log footage ungraded is the mistake this term exists to prevent. YouTube will not correct it, viewers get grey mush, and the extra range you shot for is wasted. Grade to Rec. 709 unless you are delivering HDR, which is a separate pipeline with its own standards.