What does the PF function do in Primefaces

The 

PF

 function
is a part of PrimeFaces's JavaScript API. It looks up a Javascript object that is the backbone of the JSF component on the client-side. Here is its definition (source):

PF = function(widgetVar) {
var widgetInstance = PrimeFaces.widgets[widgetVar];

if (!widgetInstance) {
PrimeFaces.error("Widget for var '" + widgetVar + "' not available!");
}

return widgetInstance;
};

PF

 is
a shortcut for 

PrimeFaces.widgets['someWidgetId']

,
which just looks-up a Javascript object in global scope, and so the Javascript object can also be retrieved using 

window['someWidgetId']

.

The PrimeFaces's Javascript API has no official documentation online, so to understand what you can really "do" with the Javascript object, you'll need to take a deep dive into PrimeFaces.

See also

"Intro
To PrimeFaces widgetVar" blog post
PrimeFaces
source code