Fiddlers call

Could someone please tell me what a fiddlers call is? It would appear to relate to a late and spur of the moment invitation to an event party etc.

In square dancing, as I understand it, couples are invited onto the dance floor and are instructed which steps to do (alaman left, dosido, etc.) by a "caller". There is also a fiddler and other musicians, and I suppose if you're on a budget the fiddler also calls the dance.