Occasionally, when reading through an interesting thread you'll find a specific post you'd like to respond to; this is when you'll use the quote feature. When viewing the post that you would like to respond directly to, click the Quote button that will appear in the top-right hand corner of the post. Click this quote button and when that is done, you'll be taken to the posting page with a quote automatically provided in the message area for you.
The quoted message will appear in a table in the message area. you can can click into the quote box on the posting page to make edits to the quote, such as removing unnecessary lines so that you can respond to a specific part of the member's post directly. Once the quote is to your liking, click below the bottom line of the text box so that your typing cursor will appear outside of the quote box. From there, you can begin typing the rest of your response.
While holding down the Alt key you can click on the content area of a post to select it. Clicking on multiple posts like this and then clicking a Quote button will let you multi-quote. The quoted messages will all show up in the message area. You can reply by typing a message after each one. This way multiple quotes with multiple replies will show up in the same post.