A <link rel="canonical" href="{url}">
tag in the <head>
of a page specifies the canonical/"main" URL for the current page. When the same content can be accessed at different URLs, it specifies the preferred URL for the page. It was originally introduced for use with search engines.
For example, https://www.smore.com/577k7, https://www.smore.com/577k7-whatever and https://www.smore.com/577k7-the-edtech-monthly all return the same page. That page has a <link rel=canonical>
field which specifies https://www.smore.com/577k7-the-edtech-monthly as the preferred URL.
See also https://en.wikipedia.org/wiki/Canonical_link_element.