It's refreshing to see someone answer a question, instead of referring a new user to research their question or linking them to an article from Google. The internet would be a much better place if people would give a brief explanation, rather than making people research themselves. 
It literally means Restoring your phone to factory settings with iTunes. After that, setting it up as a new phone, not "Restoring From BackUp".
It is a PITA, laborious process, that can result in some limited data loss if you have any Apps that don't have their own cloud backup solution. However, it is the only way to truly evaluate the performance of a new OS.