Explanation
A Resource Address is a string that references a specific resource in a larger infrastructure. An address is made
up of two parts:
[module path][resource spec]
Module path:
A module path addresses a module within the tree of modules. It takes the form:
module.A.module.B.module.C...
Multiple modules in a path indicate nesting. If a module path is specified without a resource spec, the address
applies to every resource within the module. If the module path is omitted, this addresses the root module.
Given a Terraform config that includes:
resource "aws_instance" "web" {
# ...
count = 4
}
An address like this:
aws_instance.web[3]
Refers to only the last instance in the config, and an address like this:
aws_instance.web
Refers to all four "web" instances.
https://www.terraform.io/docs/internals/resource-addressing.html