Cron is a linux utility used for scheduling tasks on your server that run in the background. The tasks are known as jobs in cron lingo and are usually scripts for automating mundane tasks.
A base installation of WordPress uses cron for checking for updates to the core and plugins/themes. In addition, plugin authors can add their own scheduled tasks.
The cron system used within WordPress
WordPress invented their own cron system, called WP-Cron, because using a real system cron for scheduling tasks was rarely available on shared hosting. On every page load WordPress checks if there are any jobs that needs to run and if this is the case it starts up these tasks in a background process. As you might have guessed this is not the most reliable system since you can not expect that jobs will run in a timely manner.
Cron the WP Next way
So to fix the inherent issues with WP-Cron we have disabled it on WP Next and given every site its own system cron. Now your sites scheduled jobs will be run reliably. And as a bonus, WordPress no longer has a need to check for jobs that need to run on every page load.