What is Whistle tone or Whistle Register

The whistle register is the range of vocal notes above E6. Whistle register is an extension of your head voice or head register. The sound in that range are similar to whistling sound and hence the name. A properly pitched whistle register tone can shatter glass, there are some videos on Youtube which you can watch.  
The ability to hit notes in this register is extremely rare.Most male singers do not have to worry about Whistle sound or register as it applies only to female singers. In male singers the same is called falsetto which you would already be singing. 
Even in female singers not everyone can achieve the scale of whistle sound or register. Some singers are gifted and have a natural extended vocal range while others work upon it and develop a high vocal range. 
Check some of the posts related to whistle register http://www.wikihow.com/Belt-Sing-in-Whistle-Register