The difference between the infrastructure described in Terraform code (and reflected in the state) and the actual state of resources at the provider, usually caused by manual changes made outside of Terraform.
In Terraform Associate it's detected with terraform plan -refresh-only, which updates the state to reflect reality without modifying the code or applying changes. A normal terraform plan would show the drift as changes to revert on the next apply.
Configuration drift is a concept that can show up on the exam. Check the full guide to see it in context.