Your Activity feed is a summary of everything that’s happened in the channels that are in your Teams list, including any recent chats. Select the filter button at the top right corner of the feed to show specific types of messages such as unread messages, @mentions, replies, and likes.
When a red circle appears next to the Activity button, you have a notification— such as an @mention or a reply in your feed. These notifications remain in your feed for 14 days. After that they expire and no longer show in your feed.