The reply is a response by plaintiff to defendant's answer. A reply occurs only when defendant has asserted a counterclaim or the court has ordered a reply.
It is important to keep in mind that "plaintiff" in this context may also refer to an impleaded party. So, if a defendant impleads a party, this new party is the third-party defendant and the original defendant is the third-party plaintiff. The third-party plaintiff must file a complaint on the third-party defendant, who then must answer. The court may order a reply to this third-party defendant's answer.