Upvoting is how you tell your classmates that they posted a good question or comment.
To upvote a question, click the question mark icon in the comment bubble. Upvoting tells the author of the question that you have the very same question that they do! This can help attract other students or the instructor to spend time answering this question, as it indicates that many students have the same question.
To upvote a comment, click the checkmark icon in the comment bubble. Upvoting tells the author of the comment that it helped your understanding. This provides some feedback to the author to tell them that you appreciate their efforts!