The Activity Feed is one of the views available on the App bar. It’s where you can quickly view other people’s recent activity that's relevant to you, as well as your own recent activity.
When you first open the Activity view, it will display the Activity Feed. This feed displays all sorts of notifications involving you, such as @ mentions in team discussions, replies to threaded conversations, likes on messages, and missed calls and voicemails.
Click an item in the Activity feed to display it.
When you select an item in the Activity Feed, that item’s chat or channel is displayed in the Content pane.