When Disqus-enabled pages are visited, Disqus uses this identifier to determine the appropriate comment thread to load. If the appropriate thread could not be found, a new thread is created. Disqus identifiers keep threads and pages associated.
Disqus identifiers are used with the Disqus embed code and with the comment count code. If you're using Disqus on a supported platform such as WordPress or Blogger, the identifiers are used within the plugins.
Identifier vs. thread ID
Disqus identifiers are not the same as thread IDs, which are what Disqus uses internally since not all sites take advantage of Disqus identifiers. All threads have thread IDs; not all threads have Disqus identifiers.