Basically, it means that the design adapts to different browser sizes — so that instead of needing a separate mobile site or mobile plugin, the content will be resized and rearranged to fit smaller screens.
And I thank you, too, Amy.
With all due respect to theme designers. I’ve tested about 20 “responsive themes” and almost none were great. I’ve also tested some premium themes that claim to be responsive and they weren’t responsive either without adding media queries and several lines of CSS. Almost all responded to screen size but most didn’t adjust fonts well, Or did OK on landscape on Android but not in portrait. We’ll have to see how this evolves. For now I’m creating responsive sites with a lot of extra effort – which is OK b/c I like to create a custom look and feel for my sites.