can be a greeting, an expression of bewilderment, an expression of over excited agreement, or nickname for a friend.
YAGOO! Wats poppin?
friend: you do the hw for today?
me: yagoo? we had hw!?!?
girl: yea sure you can have my numba
me: hey yagoo, how was your weekend?
girl: haha it was aight how was yours?