Mail by default organizes email replies as conversation threads. All email messages that are part of the same conversation appear together as a single email thread, with the first email message in the thread on top of the most recent one at the bottom.
Threading helps you have a clean focussed view of the inbox - all replies/messages pertaining to a specific conversation appear together making it easy to follow or access different conversations. Clicking on an email displays the thread in the reading pane.
The body of the most recent email is expanded and you can read it to be up-to-date on the conversation.
The previous messages are collapsed and hidden from view and the number denotes the count of emails that are hidden from view. You can click on any message to expand or collapse it
You can also filter your emails based on three categories:
1. All Mails
2. Unread Mails
3. Starred Mails
Here's a small GIF for your reference: