Can someone explain mobile-first indexing?

google indexes and ranks your site based on the mobile version of each page, not the desktop one. it's been that way for every site for a couple of years now, so it's not something that switches on, it's just how it is. what it means in practice: if the mobile view hides text, collapses a section, or drops the structured data, google doesn't see it. if the site is responsive (same html, just reflows), there's nothing to do. if it's a separate m. site or a theme that strips content on phones, that's where the problem is.