AWS Organizations is used to manage multiply AWS account.
Let me show you an example.
Let’s say we have a Master account and a sub-account.
Invite new account to your organization
Login the master account and choose AWS Organizations service.
Add a new account to your Organization by clicking the Add account button.
The account you invited should receive a notification and if accept the invitation you could see one more account in your organization.
Now we have 2 accounts in our organization.