In addition to being able to search by a specific data field you can also search by various filters like label, owner of label, batch and other criteria. This "other criteria" includes starred, un-starred, draft, unlinked media, unlinked record, linked media, linked record, public Independent Media Items, private Independent Media Items, records, media items, and whether or not the item or record is viewable in the LUNA Viewer.
Select several criteria to create an advanced search. For example, because an item has to be linked to a record in order for it to display in the LUNA Viewer, select both the "Display in LUNA Viewer" and "Unlinked media" criteria to see which items are marked as to display in the LUNA Viewer but still need to be linked to a record.
After executing a search, click on "Save Search" to save that query. This is helpful so that you don't have to build the same search query over and over. When you select "Save Search" you'll be saving this as a search filter and can choose whether or not to have this search filter display in your quicklist, and whether it should be publicly viewable.