A description item for a tag looks like this:

==nameofthetag==
A description of what the tag is about.

Updating the description for a tag is easy: just hit the (edit) button at the end of the description of a tag in the index (as you appear to have done). The description to a tag should convey, in the length of a few sentences or a short paragraph, an understanding of what the topic is. If necessary, try also to convey why it is important. Oftentimes tag descriptions copy from the introductory text of the associated help or project page.

Do not change the title of the section! This title is generated and linked to automatically, and changing it will cause the Index page to "lose" the tag description.