How to group Gmail email replies into threaded conversations
Emails that are replies to previous emails can be combined into the same thread
This presents just one line in the inbox table of contents. When this line is clicked, it opens to show all the messages in that thread.
You can control this option from a setting in Gmail
In general, it's a Reply if it's the same recipients, senders, or subject as a previous message
Technically speaking, Gmail is also looking for the In-Reply-To and References hidden email headers