Data sources in Terraform allow you to retrieve information about existing resources outside of the current Terraform configuration. Data sources are read-only and do not create or modify resources

Common use cases for data sources include:

• Fetching information about existing resources (e.g., an existing AWS S3 bucket) to use in your configuration
• Retrieving attributes or properties of remote resources to use as input values for your own resources
• Or even Github repository pull request resources
• If you want to manage these resources consider using terraform import command

The arguments can vary depending on the data source

Example

// existing vpc
variable "vpc_id" {}

data "aws_vpc" "selected" {
  id = var.vpc_id
}

resource "aws_subnet" "example" {
  vpc_id            = data.aws_vpc.selected.id
  availability_zone = "us-west-2a"
  cidr_block        = cidrsubnet(data.aws_vpc.selected.cidr_block, 4, 1)
}

Reference and check the doc for more