Word can be
both a question and an affirmation. If someone says something which you don't believe or would like further confirmation of you can say "word?" This translates to "are you seious"
It can also be used as the affirmation. Here is a sample conversation:
person A: I robbed a bank today
person B: word? (used as a question)
person A: word. (used as confirmation of truth)
see also:
for real.