A ruby annotation:
The <ruby> tag specifies a ruby annotation.
A ruby annotation is a small extra text, attached to the main text to indicate the pronunciation or meaning of the corresponding characters. This kind of annotation is often used in Japanese publications.
Use the <ruby> tag together with the <rt> and/or the <rp> tags: The <ruby> element consists of one or more characters that needs an explanation/pronunciation, and an <rt> element that gives that information, and an optional <rp> element that defines what to show for browsers that not support ruby annotations.
The numbers in the table specify the first browser version that fully supports the element.
The <ruby> tag is new in HTML5.
The <ruby> tag also supports the Global Attributes in HTML.
The <ruby> tag also supports the Event Attributes in HTML.