For the community structure nodes you can set the description using Community Admin. Click on "Edit Properties" for the node and add it in the "Description" field like here:
This will also reflect on the TKB pages with the label filter applied.
For the tag cloud pages I'm not aware of a way to apply a description apart from a custom component that would insert the meta tag in the header.
But if you take a step back: The tag cloud is aimed at humans to browse content visually. It probably isn't that well suited as a page to be indexed on a web search and appear in search results as an entry page. You might therefor think about preventing these paged from being indexed at all, e.g. by adding to your robots.txt something like
Disallow: /t5/forums/tagdetailpage
