If you have a CRM Online with 25 or more paid Professional user licenses, your subscription includes additional non-production instance. Paid is a keyword here that excludes some types of subscriptions like Partner IUR (Internal Usage Rights) benefits. The other non-obvious step, as told by none other but our guest tipster Gus Gonzalez, is enabling this instance in the administration center. To access your instance, open Office 365 Admin Center, then click Admin->CRM.
This information and more can be found in Manage Your Microsoft Dynamics CRM Online Subscription document.