See the subject line: Scroll to the top of the preview area—the subject line is shown there instead of in each message. You can also click Details in the header area of a message, or look at the subject line in the message list.
Step through a conversation: Press Option-Up Arrow or Option-Down Arrow.
Show more of a message: If a reply contains the original message but doesn’t show all of it, click the See More link at the bottom of the reply.
You can change the order of messages in the preview area. Choose Mail > Preferences, click Viewing, then select or deselect “Show most recent message at the top.”
By default, a conversation includes related messages that are in other Mark emails as read or unread in Mail on Mac