Command: force-unlock
Manually unlock the state for the defined configuration.
This will not modify your infrastructure. This command removes the lock on the state for the current configuration. The behavior of this lock is dependent on the backend being used. Local state files cannot be unlocked by another process.
https://www.terraform.io/docs/commands/force-unlock.html
https://www.terraform.io/docs/state/locking.html
Terraform has a force-unlock command to manually unlock the state if unlocking failed.
If you unlock the state when someone else is holding the lock it could cause multiple writers. Force unlock should only be used to unlock your own lock in the situation where automatic unlocking failed.

Explanation
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.
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.

Re-running init with modules already installed will install the sources for any modules that were added to configuration since the last init, but will not change any already-installed modules. Use -upgrade to override this behavior, updating all modules to the latest available source code.
https://www.terraform.io/docs/commands/init.html

Explanation/Reference: https://www.terraform.io/docs/language/providers/configuration.html