A 404 error page is displayed when a webpage cannot be accessed. It is important to optimize this page so visitors are not lost to the site. An optimized 404 page should include links to other important pages like the homepage, contact page, or FAQ page. It should also include a search box and list of top posts to help visitors find what they are looking for within the site. Optimizing the 404 page with these elements helps search engines continue properly indexing the site and helps prevent loss of visitors.