Basically, think of an entertainment area as a pile of things in one corner of a room that can play or show media of some sort.
In a room, usually if a TV is there, it's one entertainment area, and all of the various things are hooked up to it.
Let's say you have TWO TVs in a room, each one is an entertainment area. If you look, it shows up in the Rooms section of your orbiter as another room. With each EA, the Media scenarios will change to match what is in an entertainment area, while the other scenario groups, Lighting, Climate, Security, and Telecom all will remain the same.
It's an extra dimension of flexibility within the system that not many will use, but those that will ,will be glad it's there.