Last week, Google fully rolled out their new mobile friendly search algorithm. This means that for now on, if your site isn’t mobile friendly, it will affect where you stand in the search rankings. Google now marks if sites are mobile friendly when you search for stuff on your smartphone browser.
By default, most WordPress sites can be adopted to mobile screens automatically if you allowed it in your theme settings. In order for a site to be considered mobile-friendly, its text has to be readable without tapping and zooming, its tap targets need to be spaced out appropriately, and the page avoids unplayable content or horizontal scrolling.
You can test if your site is mobile friendly with this handy tool.