'Subscriptions' are a method of adding users within an event to receive notifications when a document or discussion is created or updated. A 'subscribed' user will receive a notification within the application and by email when you have successfully added a document or discussion. They will then continue to receive notifications when a comment is added to the document/discussion or is edited within the application.
The 'Subscriptions' feature is a handy way of adding the users you want to keep 'in the loop' on your document and discussion threads without them having to manually monitor document categories for important updates to the event.
It is also an important feature for you, as a 'subscriber', to keep up to date on any changes in your events.