An ARN, or Amazon Resource Name, is a unique identifier used in the Amazon Web Services (AWS) ecosystem. It helps users specify resources like S3 buckets, EC2 instances, or IAM roles in a standardized format. An ARN typically includes information such as the service, region, account ID, and resource type, making it easier to manage and reference AWS resources. ARNs are essential for permissions and access control in AWS. By using ARNs, users can define policies that grant or restrict access to specific resources. This ensures that only authorized users can interact with sensitive data or services, enhancing security within the AWS environment.