Briefly, Opera had a star in the address bar that indicated whether you had bookmarked the page you were visiting. This button offers the same functionality and more.
The star is no longer in Opera's standard skin files so this button uses the icon for the top10 button instead. When you arrive on a page that you have bookmarked the button will use the heart icon, when you haven't the button will show the add bookmark icon. When you press it the add bookmark dialogue will appear