What are the little up and down arrows on these discussions for?
Just curious :P
They're used for upvoting or downvoting comments. If what someone says is very helpful, you (and other users) can press the up arrow, up voting their comment. However, if their comment is not helpful or completely not related to the discussion at all, you can press the down arrow, down voting their comment.
Check out this link: https://support.duolingo.com/hc/en-us/articles/205125860-What-are-those-arrows-under-the-comments- It tells you what those little arrows are used for.