Put VERY simply - Prayer and fasting is a way to break the enemies hold on something or someone by building up your spirit man (through prayer and bible reading) and killing your flesh (not eating - just drinking water). The sacrifice must be done in addition to prayer and bible reading to be effective, otherwise your just starving yourself for no reason. Your heart must be clean and right before God (and in a lot of cases from my experience, God shows you areas you need to repent when you begin a fast). There are so many wonderful websites to read about this subject, i would encourage you to research it. I have experience in fasting, but I can't begin to touch surface of the important details and depths of it on here.