Our sites use markup like this in a few places:
<header class="row">
  ::before
  <h1 class="entry-title">Privacy Policy</h1>
  ::after
</header>
What does the ::before and ::after signify and how could I use them?
Our sites use markup like this in a few places:
<header class="row">
  ::before
  <h1 class="entry-title">Privacy Policy</h1>
  ::after
</header>
What does the ::before and ::after signify and how could I use them?
You should not see those in markup.
You may see them in the element inspector in various browsers developer tools.
They represent the before and after pseudo-elements that you can generate with CSS.
The ::before element inserts an html element before the given element, just as the ::after element inserts an html element after the given element. (as described here, try-it-yourself)
For example:
index.html
<html>
    <head>
         <link rel="stylesheet" href="style.css" />
    </head>
    <body>
        <p>World</p>
    </body>
</html>
style.css
p::before {
    content: "Hello ";
}
p::after {
    content: "!";
}
This will result in Hello World! on your HTML page.