Bookmarks are a way for you to store a link to things you want to access quickly.
You can bookmark activity items (posts, pictures, places, notes, etc...), groups, and users. For example, if there is a group whose activity you want to check often, you can put it in your bookmarks.
Note, however, that you can only bookmark objects that are visible to you.
No. You may view your own bookmarks, but not anyone else's; in Floopstreet parlance, the list of bookmarks has private visibility.
A bookmarked object is not affected when a bookmark is deleted.
However, you should be aware of the behavior of the mobile applications: since the detail screen for a bookmark displays the object's details, any actions are associated with the object rather than with the bookmark. Therefore, the Delete action will delete the object, not the bookmark (and also the bookmark, since the object has vaporized).
As was discussed in the previous answer, in the mobile applications you cannot delete a bookmark from its detail screen. Instead, you edit the list of bookmarks and delete rows there; in this case, the bookmark is deleted, but of course the bookmarked object is not affected.
In the browser application, you delete it by clicking the appropriate link in the bookmark's toolbar.