When any Password Vault item has a tag, a Tags row under the search bar lists every tag with the number of items it holds. Choose a tag to show only its items, or Untagged for items without one. An item with several tags appears in each of them, and its tags are shown beside its name.
To tag many items at once, select them and choose Add tag to type a new tag or pick an existing one. Remove tag removes a tag from the selected items. Tags are encrypted with the rest of each item, so Magpie cannot see them.