(slang) A greeting similar to hi.
Synonyms: oi, wotcher
Yo Paulie! How's it going?
(slang) An interjection similar to hey.
Synonyms: ahoy, oi; see also Thesaurus:hey
Yo, check this out!
Check this out, yo!
(slang) An expression of surprise or excitement.
Yo, that's crazy, but I don't remember asking.
(military slang) Present! Here!
Sergeant: Smith?Private Smith: Yo!
(chiefly African-American Vernacular) Emphatic conclusion to a statement.
Source: en.wiktionary.org