Tags are keywords or labels that categorize and group your question with other, similar questions.
Tags make it easier to filter questions, and to attract experts in the field related to your question. Tags also have important explanatory text associated with them. These are called tag wikis. There are two parts to this text; the excerpt and the full text. The excerpt is used as a popup tool-tip when a user mouses over a tag.
Users can create this text by going to the Tags page, finding a tag, clicking it and then clicking on "more information".
If you create a new tag, by adding it to a question, you are encouraged to add the explanatory text to the tag as well. Leaving it unstated can confuse users and, worse, lead to inconsistent use of the tag.
When writing tag wikis some things are important.
Use complete sentences with good grammar.
The excerpt should begin with a statement about what the tag is to be used for, not for situations in which it shouldn't be used. You can include the "do not use" phrasing, but don't put it first.
Also, if you write an excerpt, please also write the complete text for the tag as well.
And if you come across a tag with only an excerpt, consider writing the complete text as well, or at least send a note to the classroom so that someone else can attend to it. Help keep the site clean and tidy.