I'm no expert but i'll try and answer your question since nobody else has.
the $this->getRequest() is doing exactly what it says. Each time a page is loaded in the browser it is first requested by the browser and the web server serves it up. In Zend you can grab that object (header call?) and there are a number of methods that can be used. You can do a number of different things with the request at that point. The $this->getRequest()->isPost() is checking to see if the requesting browser (user) has submitted a form in this particular request, if they have then it returns true and if it hasn't then it returns false. Also you can use $this->getRequest()->getParams() to get the post and get parameters of that request.
Here are the methods that can be called on $this->getRequest()
Hope this helps!