man/woman; a person who makes you smile/laugh/happy; a person you have feelings of warm personal attachment or deep affection. Beloved person; sweetheart. Strong predilection, enthusiasm, or liking for him/her. YOUR BOO! Etc.
Someone you have an unofficial relationship with. You could be "talking" to them, you don't call them your friend, and you don't call them your boyfriend/girlfriend, you call them your "boo thang". Pretty much someone you care about/love/have feelings for.