The short explanation
Extended-gamut YCbCr allows color values outside the boundaries of the old Rec.709 space, so more saturated colors can be described. It arrived with HDMI 1.3 alongside deep color.
Where you run into it
In practice, not much any more. Content using xvYCC is rare, and the wide-gamut job is now done by BT.2020 with HDR metadata. It survives on specification sheets as a compatibility item.
It is worth recognizing the name so you do not mistake it for something you are missing. If a device lists xvYCC support, that is a legacy capability, not a modern HDR feature.
How the DR HDMI 8K handles it
The DR HDMI 8K lists xvYCC color support in its specifications alongside deep color and 3D. For current content the capabilities that matter are BT.2020, the HDR modes and the Dolby Vision data block — all of which the unit can set through its EDID tables, flags and DV settings.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.