Most websites have require certain tasks to be performed on a regular schedule such as checking for inactive users, sending email notifications, reindexing the search engine, refreshing the sitemap.xml file to improve SEO, etc. In Concrete CMS, these types of tasks are called Jobs and they can be automated to occur on regular schedules using a server cron or similar system, or run manually. There are a number of built-in jobs, and custom jobs can be added through add-ons in the marketplace or one you develop yourself.