Subscribing to a task helps you to stay notified about key updates to tasks you care about.
When someone @mentions you on a task, assigns you to a task, or you post a message on a task, you are automatically subscribed to the task.
By default, you're also subscribed to any tasks you create (although this setting can be toggled off from Personal settings → Notifications).
Managing task subscriptions
You can also manually subscribe to any task you're interested in by clicking the bell icon in the top righthand corner.
This bell icon is also where you can unsubscribe yourself from a task.
Once you're subscribed to a task, you'll receive notifications in your Inbox about it.
By default, you're notified when a task you're subscribed to has new messages, when its name, description, or assignees changes, and when it's marked as "Done" or any other completed state status.
To control which things you want to be notified by, go to Personal settings → Notifications.