HashiCorp's policy-as-code framework integrated with HCP Terraform (formerly Terraform Cloud/Enterprise), letting you define mandatory policies (e.g. "every S3 bucket must have versioning enabled") that are automatically evaluated before a plan is applied.
In Terraform Associate it's presented as HCP Terraform's governance mechanism, similar in purpose to Azure Policy or AWS SCP but applied directly to the Terraform plan before apply. Not available in open-source Terraform CLI alone.
Sentinel is a concept that can show up on the exam. Check the full guide to see it in context.