Threads
Replying to a particular message starts a thread under it, so a side discussion stays attached to what it's about instead of interleaving with everything else. In the app a thread is also called a discussion.
Starting one
Reply to the message rather than the conversation:
- Swipe the message from left to right to reply.
- Press and hold it and choose Reply.
Swiping to reply with text is what creates the thread.
Reading one
A message with replies shows "3 Replies" underneath it. To open the thread:
- Tap the replies count under the message, or
- Tap the parent message itself.
Either takes you into the discussion view, which shows the original message with everything said in response to it, and nothing else.
Why it matters for catching up
Threads are why a busy conversation stays readable. When you play new messages, threaded groups play together, so a question and its answers arrive as a set rather than scattered among unrelated messages.