Cron is a utility to execute a certain set of commands or for the execution of a script as per schedule. It can be scheduled at a certain time or when specified events occur. In WordPress, there is a plugin called WP Crontol that has the functionality of executing certain actions based on the time schedule.
In general, Crons are modules which get executed automatically or periodically. In programming terms, Crons are created as a callback function which gets called by the WordPress core on the specific time period.
Using Crons, you can program a certain set of operations to be performed periodically. For example, it can be used to send emails to users at a specific time, automatically backup the database on scheduled time, automatically clean the spam comments etc.