Note I have not included links to editors, areas to download (excepting Romlama) or other pages. My information just isn't that current.
An area, or zone, is a section of the mud world. It can be as large as a jungle or as small as a village. Usually it has creatures (mobiles), objects and rooms. It may also have shops where players can buy and/or sell objects.
Usually it has a theme (a fairy forest, a military outpost, a witch's tower) but sometimes is as large as an entire kingdom.
Adding new areas to your mud gives new places for players to explore (and kill people!)
Romlama has hundreds of areas for the asking -- all you have to do is write one new area for them. Pretty good deal!
This question came up a lot more often in the early days of muds. Originally, areas had to be written in a text editor, and since an area is formatted text, it was quite easy to mess up the format just a little and create a debugging nightmare. Anybody that was willing to go to that trouble was well-regarded.
However, now that many area editors exist and there are patches to put building capability right into your mud, quality standards have generally risen. Here are some things to consider when you decide whether or not to add an area to your mud:
There are patches now that allow adding online editing (OLC) to Rom based muds. This is such an enormous advantage that many mud implementors have added this feature. There are also some offline editors. If you foolishly want to write areas offline (which makes a debugging pain in the ass) there are mostly-accurate docs in the Rom 2.4 download.
You're better off contributing them to Romlama. That means any Romlama member will get access to them, where if the areas were just in the Rom distribution, established muds would be less likely to see them.