When you've reached the end of your 30 day trial, or used all of your Free Trial credits (whichever comes first), you will no longer be able to create new paid resources. However, your account will remain active. Your existing resources will continue to run for a few days, allowing you to upgrade your account and keep your resources before they're reclaimed by Oracle. (Note that reclaimed resources cannot be recovered—they are permanently deleted.)
Resources identified as Always Free will not be reclaimed. After your Free Trial expires, you'll continue to be able to use and manage your existing Always Free resources, and create new Always Free resources according to tenancy limits.