Every automation consists of a trigger, condition, and action. Triggers start the execution of new automation. Triggers can listen for events. To configure a trigger, click on the "Configure" button in the triggers section.
Select the trigger after entering the name of your automation and selecting the project. There are some simple triggers: Issue Created, Issue Assigned, Field Value Changed, Comment to issue created, Issue linked. You just select them without any other details.
Three other triggers are more complicated: Schedule of issue, Issue Transitioned, Incoming webhook. You need to enter more details in opened forms or even do some actions for this trigger.

Schedule of issue

This trigger is to allow users to schedule when automation will run. They can be scheduled to run every x days/hours/minutes.
When it runs it can do a JQL search to get issues or simply run the conditions and actions without any issues.
Issue Transitioned
This automation trigger should activate when any issue is transitioned through its workflow. When the From and To workflow status is blank then it will always match any transition, or specific transitions can be specified (one or multiple can be selected from a dropdown).
Incoming webhook
This is used so external tools like Zapier, IFTT, or any other external cloud service can call their unique URL to trigger this rule/automation.
It needs to support 3 scenarios: issue details are provided via the webhook, the issues are provided by running a specific JQL, or no issues are from the webhook.
Click the "Save" button after selecting the trigger and entering\selecting all values in fields. Then select condition and action. Enable the automation with the toggle button button. Automation is created and the trigger will fire after the conditions will be done on the board.