Starring documents will help you easily find important documents again when needed.
Hover the document's title in the main column. A button with the labe 'Star document' appears.
Click it:
The starred document is now marked with a star icon:
Click again on its star icon. The document won't have a star anymore.
Open the 'Starred' filter on the left:
Tick 'Starred' (respectively 'Unstarred').
Only starred (respectively unstarred) documents will be displayed in the main column:
