When you select a conversation from the conversation list, that will appear in the center pane, also known as the message thread. The message thread contains the entire history of that conversation, with the most recent message at the bottom of the thread.
Below the message thread is the message field, where you can type your message, insert files, create tasks and notes, and share events.
At the top of the center pane is the conversation header bar, which might contain some of the following important action items: