Official documentation found here: Automation: Basics
Automation rules are made up of three parts: triggers that kick off the rule, conditions the refine the rule, and actions that perform tasks in your site.
One great feature of Automation for Jira is smart values. They allow you to access and manipulate issue data within Jira. They can add significant power and complexity to your rules. For example, the smart value {{now.plusDays(5)}} references the current time and adds 5 days to it, while {{issue.summary}} will print off the summary of the issue. Learn more about using smart values.
To create a new rule:

