The Configs menu is only available to Docker Swarm endpoints.
Docker 17.06 introduced swarm service configs which allow you to store non-sensitive information, such as configuration files, outside a service’s image or running containers. This allows you to keep your images as generic as possible, without the need to bind-mount configuration files into the containers or use environment variables. Secrets is another option for storing sensitive information.
In Portainer you can add and remove custom configs for use in deployments.