The following definition of "yo" is described from the Rap Glossary which is supposedly part of an explanation of rap terms used in a corporate jingle for the company known as Asera (emphasis on the word known, they may be defunct).
Yo
Rappers accross the world use this word. Its precise meaning is vague, but the rule
is it must be used several times in each sentence. "Yo, what be up with that, yo?"
You can view the entire rap glossary at http://www.techwrite.net/rapglossery.html