In AWS either you have one organization or have none, you cannot have multiple organizations. Instead you can have multiple Organizational Units (OU) for your organization
An organization always have a management account (formerly known as "master account") and optionally multiple member accounts
Hierarchy
An Organization contains Organization Units which contains Accounts
The Root OU of an organization is automatically created and cannot be removed
Consolidated Billing
An account consolidates billing across all accounts (single payment method)
With that you benefit from volume discounts for EC2, S3, etc