What is this tag for?
please-remove-this-tag is used to help in the cleanup of high-risk problem tags.
What kind of tags are a problem?
A high-risk problem tag is defined as one that is:
Vague, ambiguous, misleading, regionally-dependent, meta, or otherwise prone to misinterpretation and incorrect or inconsistent usage.
Easily discoverable - typically composed of common keywords present in the question.
Already in widespread use, such that an immediate manual removal of all instances of the tag would bump newer questions off the front page.
Such tags are considered to be serious problems because their usage tends to become both more common and more divergent over time - even amongst experienced members.
When it is impractical to "hand-sort" one of these tags due to volume, a moderator may merge the problem tag with please-remove-this-tag in order to quickly eliminate it from the tags page and any tag suggestion boxes, as well as leave a clear and easily-recognizable marker on the question itself that it will eventually require retagging.
When and how should I remove it?
Any members with the retag question privilege may perform a retag. See the aforementioned link for specific instructions on how to retag any questions.
Some additional guidelines for this specific retag:
Pay attention to other question tags. More often than not, questions with one bad tag will have other bad tags, and/or be missing other important tags. Don't just remove the placeholder; treat it as any other retag and consider which tags would actually be most appropriate for the question.
Don't mass-retag. Limit your retag efforts according to our daily question activity; at the moment, we recommend no more than 10 retags on a typical day, less on weekends. Be mindful of other members potentially doing the same thing; if you see more than a few old questions already on the front page, let it be - the placeholder will still be there tomorrow.
Don't retag closed questions. Closed questions are the short list for eventual deletion, at which point it won't matter what tags they have. Don't add noise to the front page by bumping a question that has been deemed off-topic or inappropriate.
Prioritize questions that are already on the front page. New or bumped questions have their tags on display for the whole world to see. If a front-page question has bad tags (including this placeholder), then don't hesitate or worry about site activity - retag it now.
Check these posts for answers:
If you still have questions, don't hesitate to ask them on Meta Seasoned Advice.