YITH WooCommerce Checkout Manager is a plugin that allows you to customize the checkout fields by adding, removing or editing any of them. You can take advantage of many types of fields.
The integration with YITH MailChimp allows recording information related to checkout fields that have been added or edited through YITH Checkout Manager for a perfect integration of your store features.
Just install and activate both of them to make sure that the integration can work: no other action is required from your side.
Read more about how to set up YITH Checkout Manager in the official plugin documentation.