Cron is a regularly executed script that does the maintenance on your system. It checks system health, deletes old logs and so on.
How and when this script is executed?
By default this script is executed approximately once an hour using "fake cron". Fake cron means that the script is executed when someone visits the website. Script is not executed on each page view. It is executed no more than once an hour. If you have regular website with tens of pages and few thousands page views a day, you don't need to worry about anything. Everything will work smoothly.
If you have some special plugins that need exact time when cron jobs need to be executed, you can set up cron jobs on you cPanel or from linux command line. The file you need to launch is ip_cron.php. So you can launch cron manually just by visiting http://www.example.com/ip_cron.php. Visiting this URL executes system maintenance tasks and launches cron functions on plugins.
Testing cron jobs
You can launch ip_cron.php file quite often but it will not actually do anything if there is no job required. If you need to force system to execute all cron functions regardless that cron was executed recently, add test parameter: http://www.example.com/ip_cron.php?test=1