The idea is to provide help to user when he is filling the fields of a structured content. A description would explain the kind of information that is expected in the field.
For example, a question mark icon could be positioned near the field, with a tooltip giving explanations about what is the information to provide.

For developers, it could be a new attribute for the tag "field" in the XML description file of the structured content.
For example :
<field id="author" label="Author" required="true" help-tooltip="Give the name of the article author">
<datatype base="string"/>
</field>