The WiFi Calling feature allows you to make and receive phone calls - via your standard cellular account - if your phone has WiFi coverage, but lacks a cellular signal. This means, for example, if you are at the doctor's office and don't get a signal, but they have a guest WiFi that you can connect to, your phone works 100% just like an iPhone (and not just for texting and apps).
AT&T has made this feature available to users in some areas - and mine was lucky enough to be one. After using it all day, it has been totally flawless. I highly recommend it, and if you are in an area with good WiFi, but bad cellular - upgrade to iOS 9 as soon as you can.