Tag excerpts are the popups that tells users when to use a particular tag, but when new sites start writing their wiki excerpts, we'll often end up with something like this.
email a method of exchanging digital messages from an author to one or more recipients
That's not very helpful.
In our [tag] system, we have "tag wikis" and we have "tag wiki excerpts." A Tag Wiki is a free form wiki page that allows the community to create collaborative resource about the subject. It may also contain a list of resources of interest to folks in that subject space. A Tag Wiki Excerpt is a short summary that pops up when users are picking tags for their question. It describes when a tag should be used.
Most Common Problems:
Excerpts should describe usage, not definitions
See What should a tag wiki excerpt contain?
Tag Wiki Excerpts are not typically simple dictionary definitions of what a word means. Wiki Excerpts should contain usage guidance for when the tag should be used. It may sound a bit silly or redundant to describe the 'led-zeppelin' tag as something to be used when you have question about the band, their music, legacy, music style or history… but when you have more subtle tags that need more guidance, users are not going to know what tag excerpts are for.
Exceptions to the rule:
Sometimes the subject of a tag is so obscure, it really needs the entire space of the tag wiki just to describe what it is. Also, if a tag is an acronym, sometimes it is useful just to clarify what that acronym stands for. (e.g. ascap For questions about the organization, "the American Society of Composers, Authors & Publishers", and their influence and dealings in the music industry. (oops, I actually got some guidance in there there))
Tag wikis can become a valuable resource to this community. But too often, folks hear wiki, and they simply copy the content of Wikipedia over to this site. While perfectly legal, that is not what tag wikis are for. Tag wikis are designed to create an original and valuable resource for this community specifically. So hypothetically, something like this should be rejected:
REJECTED — This edit copies a significant amount of content from an external source. Generic descriptions such as encyclopedia articles and ad copy do not provide useful guidance; try creating something useful to this community specifically, and be sure to attribute the original author. See: How to reference material written by others.
I hope you find that helpful.