Most people are searching, at least via Google, using a mobile device. Still, many people and many ranking systems, and even some designers typically look at the desktop version of a site as the main or perhaps the best version. This can be a problem.
Pages and entire sites can look great on that designer’s 30-inch monitor but fall short on that smartphone. It is a problem for ranking sites, like Google search, which admits that when the mobile version of a page has less content than the desktop page their algorithms are not evaluating the actual page that is seen by a mobile searcher.
They have been trying out ways to make results more mobile-first for the past 5 years or so. I know that Google tells me about issues with sites and many of those issues are about mobile views. The Google Search Console search.google.com/search-console/ will be primarily using the mobile version of a site’s content to rank pages from that site. That means you need to have a mobile-first mindset.