A Magic Cookie is a unique 16 bit number that is given to each object producer (a person or organization creating new objects). The purpose is to ensure that all objects created by different people have unique identifiers. They are not used for identification or tracking purposes. You may enter your name or nickname, and if you want to be notified when new versions of The Sims Transmogrifier are available, enter your email address. Your email address will not be made available to any third party, or used for tracking purposes.
Magic Cookies work on the honor system, in order that none of the cooperative object creators will step on each others toes. You are not required to register your magic cookie, you can use any number you want. But if you intend to distribute the objects you create for other people to use, it's a very good idea. Otherwise you risk colliding with somebody else who's using that same magic cookie.