The short explanation
A fixed 60Hz display redraws on a schedule. A game rarely delivers frames on that same schedule, so a frame arrives mid-refresh and you see tearing, or the console holds it and you see stutter.
Variable refresh rate lets the display redraw when the frame is ready, within a supported window — often something like 40 to 120Hz. FreeSync and G-Sync are the PC branding of the same idea.
Where you run into it
The window matters as much as the feature. Below the bottom of the range the display falls back to fixed refresh, so a game hovering at 35fps on a 40–120Hz panel gets no benefit unless the display supports low frame rate compensation.
And VRR only gets used if it is advertised. A capable console and a capable television will still run at fixed refresh if the capability data reaching the console does not mention VRR — which is exactly what an older receiver or switch in the chain tends to cause.
How the DR HDMI 8K handles it
VRR is one of the video flags in the DR HDMI 8K's EDID controls: you can enable or disable it. Nine of the predefined EDID tables are VRR profiles, all at 8K60 or 4K120 (there are none at 4K60); some of the ten custom-default slots, such as #4, also include VRR. The OLED also reports the VRR range actually being advertised, so you can confirm what the source is being told rather than guessing.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.