When viewing a particular discussion, you'll notice that the URL is usually appended with #latest.
The interface for this feature will be different depending on the user.
In this case, the #latest at the end of the URL refers to the 'latest' or 'most recent' page of the discussion for the user who is clicking on it.
For example, if User A has read everything, this will bring them to the last page of comments, and if User B has read up to page 3, it will bring them there, and if user C has never seen the discussion, it will bring them to the beginning of the thread (the discussion / page 1).
You can try this out with a test user or by using an incognito window if your community is public.
If you would like to turn off this functionality, you may request that we disable this by setting via your CSM or Vanilla Support.