“MailChimp for WordPress” is a free Plugin that lets you sign-up subscribers on MailChimp (MailChimp is a marketing automation platform and an email marketing service) lists. You can sign up at https://mailchimp.com/ for free and integrate the API key.
To sum up
“Mailchimp” and “MailChimp for WordPress” are two separate entities. MailChimp for WordPress is a Free WordPress Plugin by “ibericode” which lets you integrate “Mailchimp” on Wodpress powered sites. We’re not affiliated nor associated with them and kept theme as recommended plugin (Not required). You can enjoy the free version(it has all standard settings in free version) or purchase it if necessary. You also need to sign up at Mailchimp.
We recommend you test free version of plugin and Mailchimp at first and proceed with paid service if deemed necessary.