404 pages are what your visitors see when they land on a page that doesn’t exist (like, say, from a broken link).
While you can control broken links on your own site, you can’t control them on other sites. That’s why it pays to monitor your 404 page traffic so you can find missing pages and, if necessary, 301 redirect them to the proper page.
While you can use Google Analytics to monitor your 404 pages, I think a simpler solution for beginners is the Redirection plugin:
In addition to monitoring traffic to your 404 pages, it also helps you quickly add the proper redirect to send that traffic to the right place.