Allows you to quote multiple post in your reply. Just click the "Quote" button on the posts you want to include in your reply. E.g. post 1,2,3, and 5.
You will see the colour of the "Quote" button has changed for those posts.

Once you're done selecting, click on the "Add reply" button at the bottom of the page. You will see in the reply section, all the posts you've selected with the "quote" button.

You're welcome.

Also, if you've selected "quote" on a wrong post, you can just click on it again and it will revert back to blue colour.