What is the rel=canonical tag?

This blog is hosted in a bunch of places:

I prefer https://jameshfisher.com. The others are old, or insecure, or implementation details. To tell Google and everyone else which version you prefer, you can use the <meta rel="canonical" tag. On every page you serve, you put a <link rel="canonical" href="..."/> element, and this tells the client where the canonical location of that page is. Like so for the page /foo/bar:

<!doctype html>
<html>
  <head>
    <link rel="canonical" href="https://jameshfisher.com/foo/bar"/>
  </head>
  <!-- ... -->
</html>