The Primary Mirror of the Site (more precisely Canonical Domain or Canonical Version) is the preferred version of the URL or domain that the site owner declares to search engines as the primary and only one for indexing and ranking purposes. This declaration is made to solve the problem of duplicate content (Duplicate Content).
Conceptually, this term refers to authority management. Since the same website can be accessed via different URL variants (e.g. http://site.com, https://site.com, https://www.site.com, https://site.com/index.html), the search engine needs to know where to concentrate link weight (PageRank). Declaring the primary mirror ensures that all authority is transferred to a single canonical version.
The most common methods for indicating the primary mirror are 301 redirects (for the entire domain) or the use of a canonical tag (rel=”canonical”) (for individual pages).