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

If you are new to infrastructure as code as a concept, it is the process of managing infrastructure in a file or files rather than manually configuring resources in a user interface.
A resource in this instance is any piece of infrastructure in a given environment, such as a virtual machine, security group, network interface, etc. At a high level, Terraform allows operators to use HCL to author files containing definitions of their desired resources on almost any provider (AWS, GCP, GitHub, Docker, etc) and automates the creation of those resources at the time of application.

Explanation
https://www.terraform.io/docs/cloud/sentinel/index.html