This allows you to Subscribe Newsletter for customers. Here is the official link with the detailed Mailchimp integration.
Login to your MailChimp account. Under the user drop-down, select the Account.
Click on Extra->API keys.
Under the Your API keys section, click on Create A Key and copy your API key which you need in a moment.
Now you have our API key ready.
After that you need to enable Newsletter from back end and also put the Mailchimp API and Mailchimp List ID to complete MailChimp subscription process.
To enter your MailChimp Credentials use below link.