Task activities show you a time and date-stamped list of each action that has been performed on the task since it's creation. You will find the activities by scrolling to the bottom of any open task. There you'll see a toggle between task comments and activities. 

If you tap or click "Activities" then you'll see a list of all actions that have been taken on the task, by whom, and when. This activity log is in chronological order, with the latest task activities listed at the top. Comments are also included in this tab.

Tapping or clicking "Comments" will show all comments that have been added to the task.