The activity feed is located on the Project Hub. It is a place where users can view a history of actions taken on a project as well as comments: 

Any time a task is worked on, a note will be posted on the activity feed: 

Comments added to the activity feed can be either public (able to be viewed by Collaborators) or internal (only view-able to users and admins). 

You are also able to select Show comments only to hide any project activity in the activity feed. 

Users can also tag their colleagues to a comment in the activity feed by @mentioning someone: