The short explanation
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 color 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 how brightness information is handled: fixed for the whole film, updated scene by scene, or, in the case of HLG, carried without metadata at all.
Where you run into it
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, gray 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.
How the DR HDMI 8K handles 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.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.