The trigger is what starts your automation, in most cases the \Spatie\Mailcoach\Domain\Automation\Support\Triggers\SubscribedTrigger will be used, this one triggers the automation once a subscriber is subscribed and confirmed.
Mailcoach ships with multiple triggers:
DateTrigger: Triggers on a date & time
NoTrigger: No trigger, which allows you to trigger the automation from code by calling $automation->run($subscriber) on a subscriber.
SubscribedTrigger: Triggers when a user is subscribed & confirmed
TagAddedTrigger: When a tag gets added to a subscriber
TagRemovedTrigger: When a tag gets removed from a subscriber
WebhookTrigger: Trigger the automation by calling a webhook