In plain language
Standard video was designed around the brightness a cathode-ray tube could manage. High dynamic range lifts that ceiling: bright highlights stay bright without crushing everything else, and shadow detail survives. It needs a wider colour space and at least 10-bit depth to avoid banding.
You will meet four formats in practice: HDR10, HDR10+, HLG and Dolby Vision. They differ mainly in whether the brightness information is fixed for the whole film or changes scene by scene.
Why it matters in your setup
HDR only looks right when the display can actually deliver it. A source told that HDR is available will send HDR, and a screen that cannot reach the brightness the content assumes has to tone-map it — sometimes well, sometimes into a dull, grey picture that looks worse than SDR.
Which HDR formats get offered is decided by the capability data. That is why HDR sometimes appears on one input and not another, or disappears when an AV receiver is added to the chain.
What the DR HDMI 8K does with it
The DR HDMI 8K lets you control HDR advertisement directly. In the EDID flags you can remove HDR entirely, or set it to HDR10, HDR10 with HLG, HDR10+ or HDR10+ with HLG. If HDR is making a picture worse rather than better, switching it off at the EDID level is a one-click test that needs no menu diving on the source.
Need the hardware?
Open the DR HDMI 8K product page on HDfury.uk.