A service that continuously evaluates and audits the configuration of AWS resources against defined rules. It records a history of configuration changes and can automatically remediate non-compliance.
On CLF-C02 and SAA-C03 this complements CloudTrail: Config records "how is the resource configured now and how has it changed"; CloudTrail records "what action was performed." Useful for compliance checks: "do all S3 buckets have encryption enabled?"
AWS Config is a concept that can show up on the exam. Check the full guide to see it in context.