The AWS SAR (Serverless Application Repository) app is a tool that helps propagate CloudFormation tags to CloudWatch log groups, which are currently not automatically tagged by CloudFormation. The app can be deployed manually from the AWS console or using various frameworks such as AWS SAM and Serverless framework. To deploy the app, users need to add a specific macro to their template or serverless.yml file, which will then run a function to process existing CloudFormation stacks and propagate their tags to their corresponding CloudWatch log groups. The SAR app is available on GitHub for developers to use and contribute to, with plans to expand its functionality in the future to support other unsupported resource types.