The application file is a YAML, which eliminated the manual intervention of human, after deploying the artifact to the production machine. The AppSpec file will only work with AWS CodeDeploy. Let's try to understand what kind of manual operation done by human is eliminated by the AppSpec.yml file. 

Let's say we have a build folder ready to deploy to the production server. While deploying this package or archive, let's say some modification is required in the production server such as relocation of some file, restarting some services, and running some scripts. These things are mentioned in the AppSpec.yml file. That's why the AppSpec.yml file is checked in within the root directory of the application, so once it gets deployed to production server along with the application code, AWS CodeDeploy will start running the AppSpec.yml file. The deployment steps are mentioned in the AppSpec.yml file, such as copying some file from one location to another or restarting some services.