A subdomain is anything that is not on the main domain. If your domain is example.com, and your blog is on blog.example.com, that content is on a subdomain.
Subdomains are much debated in SEO. Any content placed on a subdomain, and links pointing to that subdomain, don't contribute to the SEO value of the mai domain. Although technically it is sometimes relatively easier to place content on a subdomain (a blog on blog.example.com, or a help section on support.example.com), it doesn't help your SEO.Back