Bookmarks are a convenient way to remember places in your code or in your environment so that you can go back to them at any point in the future. These bookmarks are saved automatically whenever they are modified, and restored when GPS is reloaded, so that they exist across GPS sessions.
Bookmarks will automatically remember the exact location in an editor, not in terms of line/column, but in terms of which word they point to. If you modify the file through GPS, the bookmark will be automatically updated to keep refering to the same place. Likewise if you close and reopen the file. However, when the file is modified outside of GPS, the bookmark will not be aware of that change, and will thus reference another place in the file.
The menu Edit->Create Bookmark allows you to create a bookmark
at the current location (either in the editor, or the browser for instance).
All the bookmarks you have created will be visible in the
Tools->Views->Bookmarks window. Clicking on the small icon to the left
side of each line will immediately jump to that bookmark.
You can rename a bookmark so that it is easier to remember what it refers to. To do so, open the Bookmarks window, and click twice on the line of the bookmark. This will change the way the name is displayed, so that you can edit it in place. Press <enter> when you are done modifying the name.
You can delete an existing bookmark by right clicking on the line, and
select Delete bookmark in the contextual menu.