In the context of AWS (Amazon Web Services), “STS” stands for “Security Token Service.” AWS Security Token Service (STS) is a web service that enables you to request temporary, limited-privilege credentials for AWS Identity and Access Management (IAM) users or for users that you authenticate (federated users). These temporary credentials are often used to access AWS resources like S3 buckets, DynamoDB tables, or EC2 instances, without having to use long-term access keys associated with IAM users.

STS allows you to assume an IAM role, which is a set of permissions that determine what actions can be performed on AWS resources. By assuming a role, you can temporarily take on the permissions associated with that role without directly using the credentials of an IAM user.

STS supports various methods of authentication, including:

1. IAM User: IAM users can use STS to request temporary credentials to access AWS resources.
2. Federated Users: External identity providers (such as Google, Facebook, or your organization’s single sign-on system) can be integrated with AWS using STS to allow federated users to access AWS resources.
3. Identity Federation: AWS supports identity federation with SAML (Security Assertion Markup Language) 2.0, which allows you to establish trust between your on-premises identity provider and AWS, enabling federated access.

Using temporary credentials provided by STS helps improve security by reducing the exposure of long-term credentials and by allowing you to implement more granular access control for different roles and users.