Non-www describes how a domain is set to resolve on the server when the domain renders without using the ‘www’ sub-domain. The domain is either written with just the domain and the ccTLD, or it could mean that the domain is set up with some other sub-domain before it. For SEO purposes, it is important for you to be intentional about the main content on the domain. It should render on the ‘www’ sub-domain or it should not. Allowing pages to render with or without the ‘www’ creates problems, because there are actually two indexable versions of the page – the one with the ‘www’ and the one without. This can slow the crawler down, when it is crawling the site, it can mitigate and defray the value of links, and it can make site migrations much more complicated.