Used as a greeting or to attract someone's attention.
Used to express emphasis.
Today's my birthday, yo.
Used variously to attract attention, greet someone, introduce or emphasize a remark, etc.
Yo sandwich only has bacon in it. Want some ketchup on that?
(Baltimore) Third-person singular, familiar.
Yo was tuckin' in his shirt! (Stotko and Troyer 2007)