The frequency of the tone you want to play or 0 if you want to add a pause.
The duration, in mS, of how long you want to play the tone for. A value of 0 will cause the note to be played continuously until either a new note is played or you call stop().
The RTTTL string, in program memory, of the tune to be played. See http://en.wikipedia.org/wiki/Ring_Tone_Transfer_Language for more details on the specification. There are loads of web sites you can use to download free RTTTL files - Google is your friend.