While Disqus provides thread descriptions on the Site Profile page, these are scraped from the page content, and cannot be manually set in the configuration variables like the thread URL or title.
The descriptions should automatically pull and show the first text content appearing in the body of the page. However, if a post does not have body content, or the scraper is pulling undesired text into the description, you can insert this code into the <head> of your page code to manually set the description to be scraped:
<meta name="description" content="{your description here}" />
Replace {your description here} with what you'd like your thread description to read as, and this will be populated as the thread description in Disqus when the thread is created.