Terraform can manage resource dependencies implicitly or explicitly. Implicit dependencies are created when a resource references another resource or data source in its arguments. Terraform can infer the dependency from the reference and create or destroy the resources in the correct order. Explicit dependencies are created when you use the depends_on argument to specify that a resource depends on another resource or module.
This is useful when Terraform cannot infer the dependency from the configuration or when you need to create a dependency for some reason outside of Terraform's scope. References = : Create resource dependencies : Terraform Resource Dependencies Explained