Subscribing to a task means that you'll get notified any time something happens. Subscribe yourself or others to tasks that will be of interest.
You’re automatically subscribed to tasks that are assigned to you and that you’ve created, but you can choose to subscribe and unsubscribe from tasks that are assigned to teammates. Subscribing ensures you receive notifications on all task activity.
Subscribing and unsubscribing is as simple as clicking the ... button at the top of the task pane and selecting "Subscribe to this task". You can see who is subscribed when you click Edit or below the comment form when it is expanded.
You're able to subscribe others to your tasks in the same. Either create a new task, or edit an existing one, and enter the names of the users or workgroups you'd like applied to the task.