See the Wikipedia article ruby character. Basically "ruby" is referring to small pronunciation characters that appear above (or sometimes below or beside) a word. A combindation of lyric and ruby data messages might be useful to display, for example, Japanese lyrics with ruby characters in a karaoke-style display window.

You may also be interested in looking at the MIDI 1.0 specification for SMF Language and Display Extensions, which describes a way to encode a "ruby part" in a MIDI 1.0 Standard MIDI File.

I don't know how it didn't occur to me to search for it by character, thanks.