Klaviyo is a marketing automation platform focused on email and designed for ecommerce. This is a plug and play service. Once you've signed up with them, you'll just enter you credentials in the admin under: Config > Plug-Ins > Klaviyo.

• Use Klaviyo Tracking: To enable or disable Klaviyo.
• Public API Key: Key from your Klaviyo account used for Javascript Klaviyo tracking events. It's okay to share elsewhere.
• Private API Key: Key from your Klaviyo account used for all other Klaviyo tracking events. Treat this like a password and do not share elsewhere.
• Selected Mailing List Provider: Used to enable/disable mailing list portion of Klaviyo. It's possible to have analytics managed with Klaviyo, and mailing lists managed with another service.
• Historical Data: This button is used to send all order data to Klaviyo, for better analytic statistics. Otherwise data is sent as orders come in after Klaviyo is activated.

Klaviyo Feed Generator

Generating this feed is needed for Klaviyo to receive accurate product data for your store. Once generated, you'll have to tell Klaviyo where to find this file on your site, on the Klaviyo dashboard under Data Feeds.

• Data Feed File Name: Name for the data feed file.
• Next Time Run: The next time the Klaviyo feed is scheduled for generation.

See here on locating your credentials: How do I find my API keys or Site ID?