A meaningful contribution is any contribution that shows that you gave it some thought.
Questions, answers to student question, adding additional information to answers, asking for more information about certain aspects, disagreeing with aspects of an answer, correcting statements that you think are inaccurate, commenting on aspects of answers that you like but did not think about before, etc, are all meaningful contributions. You can and should discuss homework and extra credit questions with your classmates, but please do not post the answers directly.
Just saying "Yes", "I agree", "I like your answer", etc. before the module's discussion deadline are not meaningful contributions.