This is a Docker Compose file for a MongoDB container based on the mongo image. The following instructions are included:

1. Use the version directive to specify the version of the Docker Compose file format that this file uses. In this case, we use version 3.1.
2. Define a service for a MongoDB container. The image field specifies the Docker image to use for this service, which is set to mongo.
3. The restart field is set to unless-stopped. This means that if the container is stopped, it will not be restarted unless it is manually started or the Docker daemon is restarted.
4. The ports section maps port 27017 of the host to port 27017 of the container, allowing you to access the MongoDB server from your host machine.
5. The volumes section mounts a directory called mongodb on the host to the /data/db directory in the container, and another directory called mongodb to the /data/configdb directory. This allows you to persist data between container runs.
6. The environment section sets the username and password for the MongoDB server, which can be customized to your preferences.

1. Save the docker-compose.yml file to your project's root directory.
2. Open a terminal window and navigate to the directory where you saved the Docker Compose file.
3. Run the command docker-compose up to start the MongoDB container.
4. Once the container is running, you can access the MongoDB server by connecting to port 27017 on the host machine using a web browser or a REST client like curl. The username and password for the MongoDB server are specified in the environment section of the Docker Compose file.
5. To stop the MongoDB container, run the command docker-compose down. This will stop and remove the containers created by the Docker Compose file.

Note that this Docker Compose file assumes that you have already installed Docker on your system. If you haven't, you can download it from https://docs.docker.com/get-docker/.