Had no idea. I was just replying to threads with whatever smart-assery I could come up with as I caught up.
Below each post you have these two options (next to the Like-Botton):
When you hit the "+Quote" button the respective post gets marked for being quoted later on once you reach the reply-box.
For doing so click "Insert quotes" below the reply box:
It then shows you a list of all the posts you have selected where you can review them once again:
(I just selected some random posts here as an example).
When you then hit the "Quote messages" button all the selected posts will be quoted and you can write your reply to each of it in one post.
When you hit the "Reply" button instead you immediately reply to that respective quote - as if you had selected only one post with the method above.