The Collection Tracker section is meant for keeping track of the doll items that you own or that you would like to acquire.
The easiest way to add items to your collection tracker is from the Collector's Guide. There are "Add to My Collection" and "Add to My Wishlist" links on every product page. When you're looking at a list of products (for a company, brand or character), in the table view, the links will be on every line. You can edit the collection item entry to add details like where you acquired it, how much you paid, what condition it is in and so on.
In the Collection Tracker section you can view your list and sort and filter it to see how many things you own, who made them, and so on. You can also see and refine your wishlist.
To learn more about the Collection Tracker, visit the Keeping Track of Your Collection handbook page in the Help section.