The short explanation
Multicast DNS is name resolution without a name server. A device announces its own name on the local network, and anything that speaks mDNS can find it. Apple calls the wider system Bonjour.
Where you run into it
A name survives what an address does not. If the device's DHCP lease changes, its .local name still works — which makes it the most convenient way in, provided your device supports it.
Support is where it gets uneven. macOS and iOS handle mDNS natively, Linux usually via Avahi, Windows patchily. Some corporate and guest networks block multicast entirely.
Which is why the advice is always: use the name for convenience, keep the IP address as the fallback that always works.
How the DR HDMI 8K handles it
The DR HDMI 8K offers http://DR8K-xx.local, where xx is the last two digits of its serial number. mDNS can be switched on or off in the CONFIG section, and the current mDNS address is shown on the OLED info page and in the header of the web interface. The manual is explicit that only the IP address method is universal.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.