Bookmarks are saved Card Database filter searches for quick, future reference.
To make one, make sure you have a filter search in the card database that you search regularly. Once you have that checked, you hit the open book symbol at the right end of the text input bar. Then you hit New Bookmark, the Pencil symbol and then name it.
To use it again, you hit the Bookmark symbol in the search bar and hit your saved bookmark. The cards showing will now be changed to that choice. Anything you can choose in the Card Database Filter, you can have as a Bookmark. But be patient, once you select a bookmark, it could take a short time to compile them for you.
Preexisting Bookmarks are:
In Collection will display cards which are in binders set to My Precious or Trade Binder.
Not In Collection will obviously show cards you do not have in My Precious nor Trade Binder.
In Decks will display cards you have selected in your decks.
Of course, Not In Decks will show cards you don't have in a deck yet.