A “responsive” theme is one that automatically adjusts how your WordPress website displays to the size of the screen the visitor is viewing it from. For example, the navigation menu may contract itself into a clickable menu icon when your site is viewed from a phone or tablet, or images may appear above or below text instead of next to them. Sidebars may also reposition themselves so that they stack below the main section instead of next to it.

A well-built responsive theme will keep things centered and spaced so that they look good no matter what size screen they are viewed from. Note that some customizations may conflict with the automatic adjustments of a responsive theme. It is a good idea to check your site from a smaller screen whenever you make significant layout changes. If you don’t have other devices available for testing, you can also get a general idea of how the changes look by adjusting the size of your browser window.

Since web surfing from devices other than a computer has become very common, a responsive theme is usually a good choice for new sites. However, if you already have a site built in a “fixed width” (i.e., NOT responsive) theme, there are several plugins available for self-hosted WordPress that allow you to configure a responsive version of your site to resemble the desktop version, so your site can still be phone-friendly.

On WordPress.com, many themes are already responsive. Visit Appearance/Mobile in your Dashboard. If your theme is not already “deemed to be mobile-friendly,” you can check Yes next to Enable Mobile Theme, and click the Update button to activate the generic WordPress mobile theme. The generic theme is blue and white, so your content may look very different from a phone or tablet. However, this is probably preferable to your users having to scroll and left and right to read each line of text!

NOTE: There is also a WordPress theme named “Responsive.” The theme has been around for several years, since before the term “responsive” had such a specific meaning.