What is subdomain registration?

Subdomain registration is purchase of subdomain name.

Subdomain — name you give to your website. It is domain that is part of another domain.
Subdomain usually consists of three parts of text separated by dots.

Site.pro sells only .site.pro subdomains (e.g., example.site.pro).

Subdomains, except for purchased .site.pro subdomains, cannot be used for new websites (to create a new website with Site.pro website builder).
Custom subdomains can be set up only for external use, e.g. for websites located on another hosting.