When you choose to cancel your ShipStation subscription, a number of things happen...
Your subscription will be valid until the last day of your billing period, so you will still be able to use ShipStation fully until that time.
After your subscription ends...
You will be able to login to ShipStation, but you will only by able to access the Account > Subscription screen (should you want to resubscribe).
Your free Stamps.com USPS postage account will be suspended.
You will not be able to run any reports from your ShipStation account. So be sure to run any reports you need on your shipment information while the subscription is still active.
If you'd like to re-subscribe, check out this article!