Ruby is a form of superscript, commonly used in Asian text. The WebKit blog explains it well, with examples:

"A ruby annotation is a short piece of text in smaller font, written directly above or below or – with vertical text – to either side of the base text. It is most often used in East Asian typography in order to provide further information. Most commonly it shows the pronounciation of Chinese characters. Another use case is in text books, to give the foreign spelling of a native word, or vice versa. In literary text or Manga ruby is also sometimes used to specify a variant pronunciation of the underlying characters, to add some depth or twist to the normal understanding."

http://webkit.org/blog/948/ruby-rendering-in-webkit/

The CSS3 ruby spec itself is here:

http://www.w3.org/TR/2003/CR-css3-ruby-20030514/