Here's ancestry's explanation of what the Millenium File is:

The Millennium File is a database created by the Institute of Family Research to track the records of its clients and the results of its professional research. It contains more than 880,000 linked family records, with lineages from throughout the world, including colonial America, the British Isles, Switzerland, and Germany. Many of these lineages extend back to nobility and renowned historical figures. In fact, one of the things the Millennium File focuses on is linking to European nobility and royalty. A good way to have success in using this database is identify at least one Gateway Ancestor. A Gateway Ancestor is an early American immigrant who has been identified as having roots in British or European nobility. In this database there are about 300 Gateway Ancestors, or in other words, there are about 300 individuals who have proven ties to nobility or royalty. Source information is also provided in this database, making it easier to verify the accuracy of the research done. The Millennium File is a compiled source and is similar in form to other linked databases, such as Ancestry World Tree. Databases like these are great starting points for beginning your research. It is always good to find out what others have already learned and compiled about your ancestors.