Well you can change the homepage description via a site setting, which you can see here on meta if you view source
<meta name="description" content="Discussion about the next-generation open source Discourse forum software">
As for the category homepage /c/category-name, as you can see on the support category here on meta, the description is generated from the “about this category” topic for each category:
<meta name="description" content="Support on configuring and using Discourse after it is up and running. For installation questions, use the install category.">
The /category directory page inherits the same description as the homepage which seems correct to me
<meta name="description" content="Support on configuring and using Discourse after it is up and running. For installation questions, use the install category.">
The /privacy page indeed has no meta description tag, so I suppose we could add something generic like
<meta name="description" content="privacy policy for {sitename}">
but is it really worth it? I mean how many people are gonna Google for the privacy policy for a site, plus, the <title> tag is good
<title>Privacy policy | Discourse - Civilized Discussion</title>
and the primary <h1> header tag is also good
<h1 id="privacy-questions-and-answers">Privacy Questions and Answers</h1>
So I’m not sure there’s really anything necessary to do here.