If you have been working with the Salesforce platform, you will already be aware that the Metadata API format is used for the deployment of code and configuration from one Salesforce instance to another.
Salesforce DX has a different folder format than the Metadata API format. The Salesforce DX force Metadata API commands allow you to convert the Metadata API file format into the Salesforce DX format for development, and also provides a command to deploy file representations into your production Orgs or package Orgs for ISV applications.
Metadata API commands allow you to convert the retrieved source code from the Metadata API to the Salesforce DX project format. To retrieve the source code from your Org, you can use mdapi:retrieve.
If you already have an unmanaged package, managed package, or existing source code in your Orgs, you can use mdapi:retrieve to retrieve them to a ZIP format and then use mdapi:convert to convert them to DX format...