If you have WordPress blog site, you might have heard about something about Responsive WordPress Theme, the WordPress Twenty Twelve is just a fully responsive theme.
What is Responsive Web Design, simply answer: it is for multiple platforms, especially because the mobile platform is becoming more popular, the web design need to not only fit for desktop computer web browsers, but also for mobile devices such as smart phones, and even more.
About more details RWD concept, please read Wikipedia RWD page.
Now there are some nice font-end frameworks based on RWD, the web designer can build own RWD web based on those frameworks easily and flexibly. BootStrap, was the one of RWD front-end framework, which was adopted by Microsoft Visual Studio, and many WordPress themes designers, and more and more other web designers.