It means Hey You.
It is mainly used when addressing an individual in any given situation to get his or her attention.

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed

Just calling somebody out if you don't know their name.
Or expressing how dumb that action was.
Or telling people how cool the action or thing you're seeing is.

a streamlined fusion of the salutation "hey, yo" that goes over extremely well with the modern urban entrepreneur.

a response for a very suspect or homosexual comment.

often used as a gteeting in replacement of "hey yo"

This is a Nigerian name of the Yoruba dialect which means 'Joy'. It can be combined with other names.