A mobile-friendly site automatically adjusts to look and work well on phones and tablets, not just computers. Since most web traffic is now on phones, it's no longer optional.
Also known as: Responsive design, mobile responsive, mobile optimization
In plain English
A mobile-friendly (responsive) site detects the visitor's screen size and rearranges itself - stacking columns, resizing text and images, converting menus - so it works properly on a phone, not just a desktop.
Why it matters for your business
Most web traffic is mobile, and Google indexes the mobile version of your site. A site that's broken or cramped on phones loses the majority of its visitors - and ranks worse because Google's mobile-first indexing judges you by the phone experience.
When you'll run into it
Always relevant - but especially when pages look squeezed on phones, buttons are too small to tap, or text requires zooming. Test real pages on real devices, not just resized browser windows.
Run into this on your site?
Speed Optimization