A mobile sub domain is a separate segment of a site server. It is expressed with a name that occurs before the domain name. So in an example, if https://example.com is your domain name, then the mobile sub-domain might be https://m.example.com, or less common, https://mobile.example.com. These are different form mobile sub directories, which are expressed after the domain name. In either case, it is important to help Google understand the relationship between the mobile versions of the page and the desktop versions of the page, by using bi-directional annotation.