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