An overlay is the addition of another area code to the same geographic location as an existing area code. It is the preferred method of adding a new area code as it is the least disruptive to existing customers. Unlike an area code split, it doesn't require you to change your existing area code or telephone number but does require that you dial the area code and the seven-digit number for all calls.
The need for a new area code is driven by the need for additional telephone numbers, often associated with increased use of wireless devices and positive economic growth. In Kentucky, the 364 area code is being added to the area served by the 270 area code.