What are Always Free cloud services? Databases: Your choice of Autonomous Transaction Processing or Autonomous Data Warehouse. 2 databases total, each with 1 OCPU and 20 GB storage. Compute: 2 virtual machines with 1/8 OCPU and 1 GB memory each. Storage: 2 Block Volumes, 100 GB total. 10 GB Object Storage. 10 GB Archive Storage. Additional Services: Load Balancer: 1 instance, 10 Mbps bandwidth. Monitoring: 500 million ingestion datapoints, 1 billion retrieval datapoints. Notifications: 1 million sent through https per month, 1,000 sent through email per month. Outbound Data Transfer: 10 TB per month. Reference: https://www.oracle.com/cloud/free/#always-free

Reference: https://docs.cloud.oracle.com/en-us/iaas/Content/Billing/Concepts/budgetsoverview.htm

You are billed for the dedicated virtual machine host as soon as you create it, but you are not billed for any of the individual VM instances you place on it. You will still be billed for image licensing costs if they apply to the image you are using for the VM instances.

Explanation
It is collection of related resources. Compartments are a fundamental component of Oracle Cloud Infrastructure for organizing and isolating your cloud resources. You use them to clearly separate resources for the purposes of measuring usage and billing, access (through the use of IAM Service policies), and isolation (separating the resources for one project or business unit from another). A common approach is to create a compartment for each major part of your organization. For more information, see Overview of the IAM Service and also Setting Up Your Tenancy.
To place a resource in a compartment, simply specify the compartment ID in the "Create" request object when initially creating the resource. For example, to launch an instance into a particular compartment, specify that compartment's OCID in the LaunchInstance request. You can't move an existing resource from one compartment to another.
To use any of the API operations, you must be authorized in an IAM policy. If you're not authorized, talk to an administrator. Ifyou're an administrator who needs to write policies to give users access, see Getting Started with Policies.