In Gmail, all messages that are associated with each another are known collectively as a conversation. A conversation can be expanded to show all individual messages within that conversation. Conversations are sometimes called threads in other applications.
Click on a conversation in the Inbox to view it.
If a message does not have any replies, it is shown as a single message:
If a message has had replies, the message and its replies appear together in a single conversation. When the message is opened, all of its replies are shown in order, with newer messages at the bottom of the list and older messages at the top.
Conversations display the number of individual messages within them, along with the name of each message sender.
Any conversations that contain an unread message show in bold in the Inbox.
To view messages within conversations:
View a particular message by clicking on its header (i.e. the name of the sender followed by a snippet of message text)
Click on Expand All towards the right-hand side of the screen to view all of the messages in a conversation
Click on Collapse All to show only the latest message in a conversation
To work with a particular message in a conversation (e.g. to reply to a message or forward it to someone else), first expand the message and then click on the Ellipses to the right of it.
The date or time that the message was received shows at the end of the message header in the Inbox. If the message was received within the last 24 hours, it will show the time of receipt; after 24 hours it will show the date. Position the mouse over the date or time to view both the date and time in full.
If you would prefer not to see your messages grouped by conversation, and would rather see each individual message listed in the Inbox, you can disable Conversation View as follows:
Click on the Settings cog near the top-right corner of the screen
Scroll to the bottom of the Quick Settings pane
Uncheck the Conversation View option