This part of the screen is displaying the list of all messages in the currently selected folder. The number of messages displayed in the list and the total number of messages in that folder is stated right below the list. According to your settings, this list only displays a limited number of messages at a time. Use the allow buttons below the list to browse through the multiple “pages”.
The application periodically checks for new messages and will update the display if new emails arrived. You can force this check manually by clicking the Refresh button in mail view the toolbar.