Replies are attached to forum topics. You can either reply to a topic from backend or front-end.
Click on the topic edit item link. The, click on the "Reply" button. This will take you to the new reply layout with topic ID already filled in.
Alternatively, you can create "New" reply and enter the topic ID manually.
The reply form is displayed on the single topic view. If the user has access to reply, the form will be displayed.