Explanation:
You can develop custom AWS Config rules to be evaluated by associating each of them with an AWS Lambda function, which contains the logic that evaluates whether your AWS resources comply with the rule.
You associate this function with your rule, and the rule invokes the function either in response to configuration changes or periodically. The function then evaluates whether your resources comply with your rule, and sends its evaluation results to AWS Config.
Reference: http://docs.aws.amazon.com/config/latest/developerguide/evaluate-config_develop-rules.html