Import will find the existing resource from ID and import it into your Terraform state at the given ADDRESS. ADDRESS must be a valid resource address. Because any resource address is valid, the import command can import resources into modules as well directly into the root of your state.
Terraform is able to import existing infrastructure. This allows us take resources we've created by some other means (i.e. via console) and bring it under Terraform management.
This is a great way to slowly transition infrastructure to Terraform.
The terraform import command is used to import existing infrastructure.
To import a resource, first write a resource block for it in our configuration, establishing the name by which it will be known to Terraform. For example:
resource "aws_instance" "import_example" {
# ...instance configuration...
}
Now terraform import can be run to attach an existing instance to this resource configuration:
$ terraform import aws_instance.import_example i-03efafa258104165f
aws_instance.import_example: Importing from ID "i-03efafa258104165f"...
aws_instance.import_example: Import complete!
Imported aws_instance (ID: i-03efafa258104165f)
aws_instance.import_example: Refreshing state... (ID: i-03efafa258104165f) Import successful!
The resources that were imported are shown above. These resources are now in your Terraform state and will henceforth be managed by Terraform.
This command locates the AWS instance with ID i-03efafa258104165f (which has been created outside Terraform) and attaches its existing settings, as described by the EC2 API, to the name aws_instance.import_example in the Terraform state.
As a result of the above command, the resource is recorded in the state file. We can now run terraform plan to see how the configuration compares to the imported resource, and make any adjustments to the configuration to align with the current (or desired) state of the imported object.
https://www.terraform.io/docs/commands/import.html

Terraform state captures all dependency information, both implicit and explicit. One purpose for state is to determine the proper order to destroy resources. When resources are created all of their dependency information is stored in the state. If you destroy a resource with dependencies, Terraform can still determine the correct destroy order for all other resources because the dependencies are stored in the state.
https://www.terraform.io/docs/state/purpose.html#metadata

Structural type of variable allows multiple values of several distinct types to be grouped together as a single value. They require a schema as an argument, to specify which types are allowed for which elements.
https://www.terraform.io/docs/configuration/types.html