Many email programs (clients) and web mail systems have had the option to bundle your emails by "thread" or "conversation" (as opposed to discreet emails that show up as they arrive).
This is where the only email you see in your inbox is the last one you received within the context of any particular set of communications with your recipients.
This should not be mistaken for the replies of emails that you see at the bottom of any particular running email correspondence you may have with someone (though it factors into how "conversation mode" works).
Confused? Yeah, conversation mode sucks.
In the image below conversation mode is on. You can see the bracketed numbers afters the names of people in the conversation e.g. John, Callum, Me (4).
That's how many emails and people are hidden within that one line item i.e. the whole conversation between those people.
In the image below conversation mode is OFF. You can see each email only has one sender. If you click on it you only see that one email, not everything that has ever happened in that to and fro.
I've never liked it so I've always turned it off where I can.
Until recently the phone app version of Gmail, my favorite email app on my phone despite its shortcomings, only displayed emails in conversation mode.
However, magically my updated Gmail app noticed that I had conversation view turned off everywhere else and finally decided to bend to my will.
So now Gmail on my phone is working exactly how I want it to.
Wonders will never cease.