There are a couple ways you can quote a post when replying through the community. The first is to use the quote icon, which is the word bubble icon on the left:
The other way to quote is to highlight a portion of the post you are replying to before hitting the reply button. That will quote just the section you want to highlight:
After you hit the reply button, you’ll see the quoted portion in the text box where you compose the rest of your post:
Quoting is a helpful way of highlighting which post you are replying to, especially when you want to reply to one section of a longer post.