What is an overlayed area code?

In North America (mainly the United States and Canada), certain regions served by area codes experienced rapid expansion due in part to computer dial-up modems and the adoption of mobile phones.  Instead of splitting the regions into different area codes, regions would be assigned a second, overlaying area code.  Examples of overlayed area codes are:

Miami, FL: 305 and 786

Los Angeles, CA: 818 and 747

San Francisco, CA: 415 and 628

This means to get a "local" number you do not need to search for just one area code but can look for a secondary, overlaying code.  Wikipedia contains a list of overlayed area codes here.