You can optionally define multiple configurations for the same provider, and select which one to use on a per-resource or per-module basis. The primary reason for this is to support multiple regions for a cloud platform; other examples include targeting multiple Docker hosts, multiple Consul hosts, etc.
To include multiple configurations for a given provider, include multiple provider blocks with the same provider name, but set the alias meta-argument to an alias name to use for each additional configuration.
For Example
# The default provider configuration
provider "aws" {
region = "us-east-1"
}
# Additional provider configuration for west coast region
provider "aws" {
alias = "west"
region = "us-west-2"
}
The provider block without alias set is known as the default provider configuration. When alias is set, it creates an additional provider configuration. For providers that have no required configuration arguments, the implied empty configuration is considered to be the default provider configuration.
https://www.terraform.io/docs/configuration/providers.html

Reference: https://www.terraform.io/docs/language/values/outputs.html

The terraform apply command is used to apply the changes required to reach the desired state of the configuration, or the pre-determined set of actions generated by a terraform plan execution plan.
https://www.terraform.io/docs/commands/apply.html

Explanation
The local backend stores state on the local filesystem, locks that state using system APIs, and performs operations locally.
terraform {
backend "local" {
path = "relative/path/to/terraform.tfstate"
}
}
https://www.terraform.io/docs/backends/types/local.html

Explanation
As your Terraform usage becomes more advanced, there are some cases where you may need to modify the Terraform state. Rather than modify the state directly, the terraform state commands can be used in many cases instead. This command is a nested subcommand, meaning that it has further subcommands.
https://www.terraform.io/docs/commands/state/index.html