Assuming GET / HEAD / POST, its parameter time, otherwise ignore this. If you check the 'Fuzz The Datas' box, the parameters value will be copied thousands of times to....well...fuzz the data. If the paramater values are left blank, random datas will be used straight outta /dev/urandom.
POST reuqests include a field named 'POSTDATA' which contains the encapsulated form fields and can (but not always) include a content length. It looks just like GET parameters in the URI except that instead of each field being separated by an ampersand '&' they are separared by the semicolon character ';' ie;