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.

designed to solicit opinions or best-practices on a particular topic, with the goal of reaching community consensus.
You need help with the use of one or more of the site's features.
For discussions related to some particular question posted (or to be posted) on the main site. Do not use if your intention is to discuss/ask about a general issue, and you merely linked some question…
For discussions and questions on the appropriateness (for the main site) of a question or a certain type of question.
96 questions
Indicates that a feature request has been implemented, a bug has been fixed, or another type of request has been processed.
73 questions
Bug reports and suggestions for the MathJax system used to compile basic LaTeX code on the site.
68 questions
For questions about the design of MathOverflow. This includes bug reports and suggestions about the user interface.
56 questions
aimed for discussions about etiquette: how to behave on the site and the chat.
52 questions
The process of migrating off-topic questions from one Stack Exchange site to another.
49 questions
Questions and discussions on and related to the reputation point system used on the main site.
48 questions
Questions related to the closely related website, https://math.stackexchange.com
47 questions
For questions related to closed questions (and on-hold question) and also process of closing in general.
30 questions
for questions deleted from the site, or in a deleting process. The tag 'specific-question' should also be used when referencing a specific question.
a way to get additional attention for a question by offering some of your own reputation for great answers.
30 questions
27 questions
Questions specifically about the process of asking questions on this site.
26 questions
Questions dealing with moderator actions, pages, abilities or decisions.
26 questions
Questions about sizes and other quantities of MathOverflow, e.g., number of questions, GB/day transferred, voting statistics, etc.
25 questions
For questions about the flagging process, how flags are handled, etc. Flagging is the process of bringing a post to a community moderator's attention for any reason.
22 questions
21 questions
awarded to encourage and incentivize positive community actions within the site.
the most frequently asked and answered questions on Meta.
19 questions
For questions on usage of links at MathOverflow (both links to various parts of this site and to other sites).
A tag for questions about the community wiki feature (regardless of whether or not the questions themselves are community wiki).
18 questions