A Zimbra Web Client mailbox offers two options for viewing the messages in your mailbox. See below for these 2 methods.

Message View - Messages here are displayed by date. This is the traditional way messages are viewed.

Conversation View - The conversation view displays your messages grouped by subject. Because all messages in a conversation have the same subject, the subject displays only once in the conversation view pane. All messages related to the conversation are displayed, even if they are stored in different folders.
To switch between the views simply go to your inbox, Select View, and select your desired view.