Cron is the system that runs periodic background tasks in Drupal, like checking for module updates, sending queued emails, and running scheduled content publishing. Modules register tasks through hook_cron, and they execute when cron runs.
Cron can be triggered in several ways. Drupal has automated cron, which runs during regular page requests. This works but can cause slow responses and is not reliable if the site has low traffic.
The recommended approach is to set up a system cron job on your server that runs drush cron at regular intervals, typically every 15 minutes to an hour depending on needs. This ensures cron runs reliably regardless of site traffic and keeps the impact away from user requests. Specialised Drupal hosts handle this for you. For self managed hosting, configuring cron properly is one of the essential setup steps.