Mobile SEO is the practice of making sure your website works properly, loads quickly, and displays correctly on mobile devices — and that the mobile version has everything search engines need to index and rank it fairly.
Because Google uses mobile-first indexing, it crawls and evaluates the mobile version of your site as the primary version, even for visitors on desktop. A site that looks great on desktop but is broken or stripped-down on mobile is effectively showing Google its weaker side.