a cronjob (from chronos meaning time) is an automated task that occurs when the system clock hits a date and time
they are kept in a crontab (cron table) and are written with a number representing minute-hour-monthday-month-weekday-command
e.g.
45 23 * * 6 echo hello world
would run at the 45th minute of the 23rd hour of the 6th weekday (11:45pm on a Saturday) and print “hello world” to the console
it was invented in 1970 for Unix systems (like Jurassic Parks hacking scene), and is still in use today in most servers*
*don’t come after me with specifics, trying to ELI5 here.
that’s why RACK is better than SSC