A Bookmark is a feature that allows users to mark a favorite page in a Module and save it to a list.
If you have access to Bookmarks you can find the bookmark tool at the end of the 'breadcrumb address' as shown in the image below.
A small dialog will appear give you the option to add a title, tag and comments. The Tags menu is combination of text box and drop-down menu. You can add new tags either by typing into the text area, or selecting Add Tag... You can select any number of tags.
A list of your bookmarks can be found under the bookmarks tab on your home screen.