What is this site?

As defined in Best Practices for Speeding Up Your Web Site, this is a domain operated by discourse.org for quickly serving static assets that make up hosted Discourse forums:

1. We use a lightweight, minimal web server for serving simple static files as quickly as possible.
2. Using a different domain name increases the number of parallel downloads that the browser can perform.
3. Using a different domain name means browser cookies are not sent or attached to requests to this domain, reducing traffic and roundtrip times, and increasing cacheability.
4. The files are centralized (served from a single domain) and static (they do not change often), and thus gain maximum benefit from a CDN (content delivery network) which can send those files from the internal node geographically closest to the user requesting them.

What is served from this domain?

Static assets necessary for a discourse.org forum to operate, such as site logos, JavaScript code, and images in posts. If you view source on the hosted Discourse forum page and search for "CDN", you'll see exactly which assets are served from this domain.