A Parent Room is a room which has been set parent ok, and which contains code, exits and/or attributes which the owner of the room intends to make available to all it's child objects.
Parent rooms are commonly used either as zone master rooms, or as a means of encouraging or enabling extra features in a room's description.
As a zone master room
A zone master room operates very similarly to the MUX master room. As part of the command evaluation process, the server searches all objects in the zone master room, and the zone master room itself, for exits and commands.
You should be as careful about what is put in a zone master room as you are about the contents of the main MUX master room.
As a description tool
In games with a time or weather device, a coder or code/builder can create an @desc which switches the displayed description based on the current season and weather. This desc is installed on the Parent Room.
A player building their own home @parents their rooms to the Parent Room, and is shown the default description, which says something like:
To configure your room description, use the following attributes: &base-desc here=Unchanging elements of the room. &summer-desc here=What your room looks like in summer. &autumn-desc here=What your room looks like in autumn. . . .
Common options include seasons, day and night, and weather (eg: sunny, cloudy, raining, snowing, storm).