Quick Reply is a button located beneath the last post. If you click on the button, it brings up a reply box where you can type plain text. When you're done, you can press the "Submit" button. Or you can press the "Full Editor" button to bring up the full, normal editing window. Quick reply is great when you either know your BBCode by heart or just want to make a text-only post.
-Brian
Director of Tax Accounting Methods & Credits
SourceAdvisors.com
Opinions my own.