For a primer on what an area code is, read Wikipedia's article about Area Codes. The article you are reading now should cover anything else that is beyond the scope of the Wikipedia article, which would especially include anything that is not encyclopedic in nature.
Area Codes were artificial barriers that seperated bbs scenes from one another. Some BBS users, known as LDers, would break out of their local Area Code by paying long distance chargers to connect to other BBS Scenes in other area codes. Some BBS messaging networks, such as fidonet connected atleast part of the disprate BBS Scenes together by spanning area codes through gateways BBSes that connected the area codes together.