1988 Ford Festiva

My Festiva has started running bad, stallling and at times will not even idle. Last summer a mechanic found a burnt fusible link and replaced it. Within a couple days it blew again. I thought it was for the AC, which doesn't work so I wasn't too worried. Now I have been told it is for the 'ptc carburetor heater'. Is this a replaceable part, or must the carb be replaced? What is a carburetor heater anyhow? It has been cold and wet weather if that makes any difference.

Thanks