The short explanation
Dynamic Host Configuration Protocol. A device joins the network and asks for an address; the router picks a free one and lends it out for a while, along with the subnet mask and gateway.
Where you run into it
The word to notice is lends. A DHCP address is a lease, and it can change — after a power cut, a router reboot or a long absence. For something you visit occasionally that is fine. For something a control system talks to by address, a change means the automation silently stops working.
Two ways out: a static address configured on the device, or a DHCP reservation on the router so it always hands out the same one.
How the DR HDMI 8K handles it
The DR HDMI 8K uses DHCP by default and shows the address it received on its OLED info page. You can switch DHCP off and set a static address, mask and gateway in the CONFIG section — and if a wrong static address ever locks you out, DHCP can be switched back on or a factory reset performed from the OLED menu.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.