A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.
For questions regarding user names (display names) and what is allowed in them.
Times a question has been seen by eyeballs, or a profile has been looked over
For questions about the 'follow questions and answers' functionality, where you receive updates about a post as if you were the post owner.
For questions about exporting and printing MathOverflow posts.
The Stack Exchange Data Explorer (SEDE) allows you to create and run queries against copies of the databases of popular Stack Exchange sites.
for sandbox questions, which allow people to write answers meant for testing of a particular issue, or a draft for a later answer.
For questions about the various RSS feeds provided by Stack Exchange sites.
an open standard that allows users to log in to many websites with a single set of third-party credentials.
Questions about the hot questions list which appears on the main page and—truncated—on various sites in the Stack Exchange network (but not on this site), as well as its effect on MathOverflow.
Questions that have been closed as duplicates may sometimes be merged by moderators. One question is retained as a stub while all answers appear on the other.
the block of links that appears near the top of the right-hand sidebar on most pages, consisting of featured posts, events, blog entries, and hot meta posts.
Protected questions have the additional restriction that users with low reputation are not permitted to answer the question.
for questions regarding unregistered users.
For questions involving the "Ignored Tags" specified in a user's Preferences. Questions with these tags are dimmed or removed entirely from question lists.
For questions about MathOverflow newsletter - possibility of getting an email with top posts.
The undisclosed algorithm through which all new questions are passed to determine if they can be posted on SE.
For questions about various userscripts and bookmarklets.
For questions related to how the site looks, feels and behaves when its responsive design comes into play
for discussing platform-wide changes to StackExchange, especially how they impact the MathOverflow site.
currently under consideration, or needs further investigation.
Indicates that the post shares product or configuration change concepts during the Discovery phase. Open to receiving feedback from the Community preceding implementation.