Content items in Orchard can be categorized with tags, and the list of items tagged with a given keyword are accessible via a front-end ~/tags/tag-name URL. The browser navigates to the URL for a particular tag when that tag is selected by the user.
To edit the global list of tags in your site, go to the Manage Tags screen in the Orchard dsahboard by selecting Tags from the navigation section.
Here you can edit the tag keywords and remove tags. When you remove a tag, it is removed from all content items that share that tag. When you edit a tag, it updates all content items with that tag to the new keyword. You can also collapse tags by renaming a tag to the same name as another tag. This is useful for cleaning up your tags so that they are consistent throughout your site.
Clicking on a tag keyword in this screen will display the list of content items that share that tag.