An organization is below the user, the highest entity level.
Everything that you will create will be done within an organization: credentials, teams, projects, etc. A user can exist without one, but s/he will not be able to perform any actions if s/he is not part of at least one organization.
You can create and be part of multiple organizations, however they cannot communicate together, which means that if you had created some teams or credentials in one, you will not be able to access them in another one.
The deletion of an organization can be done, only after all the projects have been deleted. The reasons behind this, is that the infrastructures handled by the pipelines' projects cannot be destroyed upon organization's deletion.
Which is why customers have to destroy each of their infrastructure managed by projects, and then projects to avoid having unreachable infrastructure that still generate costs on your cloud provider.