Originally "word to the mother." Used by African Americans as a salutation. It means "Give due respect to the motherland from which we came." Can be used in the same context as "Keep it real." or, also, can be used to literally express respect to the motherland and African-American customs and traditions.
Man 1: I'll be seeing you man. Keep it real.
Man 2: Word to the mother.
Man 1: Word.