www (also W3) is the abbreviation for World Wide Web the system of interlinked hypertext documents commonly referred to as “the web.” It is also often used as a prefix to the domain name in URLs such as www.seocoaches.com. It is common practice to map the www prefix to the non-prefixed domain name so that https://www.seocoaches.com and http://seocoaches.com point to the same site.
To insure that search engines do not attempt to index both the www and the non-www pages, SEOs generally recommend that one version of the URLs should should be redirected to the preferred URL. For example, on this site http://seocoaches.com is redirected to https://www.seocoaches.com. This is accomplished with a 301 redirect inserted into the .htaccess file in the domain root.
See also: .htaccess, 301 redirect