This is a special tag that is designed to be used only by the system for questions that have had all of their other tags removed.
All questions in the system must have at least one tag. Normally, it is not possible to submit new or edited questions that do not have tags; the system will reject the submission until a valid tag is applied.
There are a few unusual situations that can cause this tag to be applied. When a tag has existed for six months and only been used on one question, and no tag wiki has been created for the tag, the system deletes the tag. When the community decides that a given tag can never be appropriate — for example, because it is misspelled or is off-topic — moderators may remove the tag from the site en masse.
When one of these deletions removes the last tag from a question, the system adds the untagged tag to that question so that the "all questions must have at least one tag" rule is not broken.
You can help clean up some of these questions! If they're good questions, edit them to remove the untagged tag and add more descriptive tags. If they're not so good, use the usual tools: downvoting, flagging and voting to close/delete.