Clicking a tag on a card will open up the edit view of that tag.
From this view, you can: rename, change color and delete the tag everywhere in the organization. If you enable the Tag management app, you also get access to the limited visibility and merge tags settings via the Advanced menu.
If no users are added here, the tag is visible to everyone in the organization. Clicking Merge into... gives you the option of merging a tag into another.
Enabling the app will also give you access to the administrative tag management view.
From this view, you can: get an overview of all the tags in the organization, easily edit tags, see usage statistics, see who can see what and limit tag creation/deletion to administrators only. Clicking a tag from this menu will give you access to all the same editing features as clicking the tag on a card.