A hard stop is an immovable end time: “I have a hard stop at three” warns the room the speaker leaves at three regardless of where the agenda is. It is the useful cousin of the meeting that drifts because nobody named an ending.
Hard stops are calendar hygiene: they force prioritisation, protect the next commitment and stop rooms overrunning into the next booking. Room systems help enforce the physical version, since a display showing the next meeting outside the door is the politest hard stop there is.