Display protocols are the command languages that let software control the screen as hardware (power state, input selection, brightness, volume) rather than only the content on it. RS-232 over serial, CEC over HDMI, and vendor-specific network protocols like Samsung's MDC or LG's SuperSign are the common ones.
They matter for the unglamorous half of running a network. Screens that power down overnight on a schedule instead of running 24 hours, brightness that tracks ambient light rather than blazing in an empty building at 3am, a display that can be power-cycled remotely instead of dispatching someone with a ladder. Those are energy cost, panel lifespan, and support-visit savings.
The caveat is that protocol support is inconsistent, undocumented and changes between firmware revisions of the same model. A command that works across a fleet bought last year may not work on the units bought this year under the same part number. Test on the exact hardware being deployed, and treat "supports RS-232" in a spec sheet as the beginning of the conversation.
CEC in particular is notorious: nominally standard, implemented differently by every manufacturer, and prone to waking devices nobody asked to wake.