Cron Service

Cron Service is MailPoet's agent which triggers cron jobs on customers websites. Cron jobs are periodically executed tasks.

The way it works is when a customer buys MailPoet 2 Premium and activates it, along with activation there is an option to have MailPoet Cron service continuously ping the site, so WP tasks that rely on WP cron get executed.
Each WP site has /wp-cron.php file which is dedicated to running cron tasks for the site. It tends to be triggered by visitors visiting the site, or some sysadmins set up a Unix cron to periodically call the script.
So MailPoet Cron service does just that. Every 5 minutes it will send an HTTP request to site's /wp-cron.php file and wait for it to complete.

And since MP2 relies on WordPress Cron to send emails, doing so will help MailPoet 2 send emails faster, without having the site rely on having a continuous traffic of visitors to trigger sending.

Here you can read how to configure this service.