A file automatically generated by terraform init that pins the exact versions and checksums of the providers used, ensuring all team members and CI pipelines use exactly the same versions.
In Terraform Associate the good practice tested is keeping it under version control (git), just like a package-lock.json. terraform init -upgrade updates the lock file while respecting the version constraints defined in required_providers.
.terraform.lock.hcl (dependency lock file) is a concept that can show up on the exam. Check the full guide to see it in context.