You can quote someone else's post or comment or a section of it in your reply.
This is useful for other people to see what someone else originally said and to see what has been going on in the discussion.
How to add a quote:
1) Open the page that has the post from which you want to quote.
(If you want to quote only a section of the comment, highlight the section you want to quote as if you were going to copy or paste it.)
(This image is for quoting a section only)
2) Click/press reply to the post like in the image below.
(If you want to quote just a section, press "quote reply" like in the image above instead.)
3) In your reply, you will see a little speech bubble icon. All you have to do is press it to add the quote into your reply