A way to consolidate and canonicalise your website addresses.
If you have a page that can be reached by multiple addresses, adding a canonical tag will allow you to not only inform browsers which one is the "real" one, but also allows you to avoid duplicate content penalties.
There are many ways to enforce canonicalisation, but the simplest is with a <link> tag.
Example:
<link rel="canonical" href="https://northrook.com/" />In this example, if visiting the site via www.northrook.com, it would inform the browser that we intend only to use the non-www version, northrook.com.
Note: There are many ways to help enforce this, from server-side actions to DNS configuration.
If you'd like to learn more in detail, we recommend reading Google Search Central's in depth article on Consolidating duplicate URLs.