traditionally, in the hip hop cultural world, the word "up" succeeds the word "word". this combonation, of what seems like two unrelated words, actually is a form of acknowledgement, or apporoval. Some synonyms inculde, "I concur." and "Yes, i agree with that"
"Gee George, your lawn looks very nice today"
"Word up"
"Oh wow, Mary, you really knew what you were doing when you decoratedthe great hall"
"Word up"