A cleansing diet would be rearranging your entire diet, for a short time or forever, to emphasize cleansing
toxins out of your colon
, liver, and other organs. The reason you read about fasting a lot is that it's the best cleansing diet there is -- you take in no toxins while your body has the opportunity to clean itself out in the meantime. How long to fast is controversial, but when first starting out it's usually recommended to do one day, then move on later to two, etc.