Favourites allows you to have an address or phone book of your most commonly used numbers to save you having to remember them. This feature is account-wide, not just applicable to a number.
Select any number then click Favourites at the top of the screen.
Here you can add all your favourite contacts.
Creating a Favourite means that when you are on any of the features like Divert, Menu and Queue you can begin to free type on both the description or number section and it will prepopulate the details for that saved contact and add them easily as a target.