The subscription of you plan is a recurring payment setting. It ensures that your account will be auto renewed when your last payment is due to keep your account active. However, sometimes you might need to cancel the subscription of the current plan and subscribe to a new plan.
Cancelling the subscription may not disable your account immediately, it just cancels the recurring payment and your account won't be automatically charged and renewed. For example, the subscription is cancelled on Sep 20th 2013 but your next billing date is on Oct 31th 2013, then your account stay keep active till Oct 31th 2013 even if you cancel the subscription.
Cancelling the account means that the account will be immediately disabled with no ability to log in or visit.