What are the threads in Slack?
Threads help you create organized discussions around specific messages. They let you discuss a topic in more detail without adding clutter to a channel or direct message (DM) conversation.
How do I mark all threads as read in Slack?
Mark messages as read
Click Mark as Read to clear a channel's messages when you've read them. You can also scroll to the bottom of the Unreads view and click Mark All Messages Read.