Mobile usability is the sum of properties that determine whether a website can be used comfortably on a phone. Since indexing takes place primarily based on the mobile version, it is also a matter of visibility, not just comfort. Check several specific things. Text must be readable without zooming. Clickable elements must be large enough and far enough apart that they cannot be tapped by mistake. The page must not be scrollable sideways. Content must not be covered by consent banners or pop-up windows immediately after loading. And the content of the mobile version must be the same as on desktop, including texts and structured data. Test on an actual device, not just by narrowing the browser window.
See also: Core Web Vitals, Above the Fold, Form optimisation.
