Subscriptions are a way to keep track of different Questions and/or forums. You can choose how you are notified about updates.
Or if you don't want to manually subscribe to a Question you want to follow you can turn on the Option 'Notification for Questions you've posted to, watched or marked as favorite' in the E-mail Notification Preferences Page inside the User CP.
How to watch a Question?
To subscribe to a Question, click the 'Options' link at the top of the list of posts then click 'Watch this Question'.
How to watch a forum?
To subscribe to a forum, click on the 'Watch Forum' link bellow the list of Questions.
How do I manage my Subscriptions of Watched Questions and forums?
You can see the List of 'Watched Forums' and 'Watched Questions' from the 'Email Notification Preferences' Page of your User Controp Panel. There you can select which subscriptions you wish to delete by Clicking on the Unwatch button.