The topics appear below the top row of followed channels and essentially act as filters for content that you subscribe to.
The default setting is “All,” which lists all videos from channels you’re subscribed to, from newest to oldest. Then there’s “Today,” showing all the videos published within the past 24 hours.
The other topics include “Continue Watching” (videos you started watching but haven’t finished yet), “Unwatched” videos, “Live” videos (includes livestreams as well as YouTube Premieres), and “Posts,” which shows only community posts.
Google says that tests showed that these topics encouraged more subscribers to come to their subs feed daily to look for new updates. That said, there’s no requirement to use them, and user feeds will continue to show in reverse chronological order in the traditional way if the topics buttons are left untouched.