When setting up a trigger, you can indicate how many 'runs' your contacts can go through, once or multiple times. In this article, you can read more about this.
When you choose 'Once', your contacts can only start the automation via this trigger once. For example, if someone enters the automation through the trigger 'Fills out a form' and fills out the form again after a while, they cannot go through the automation again.
If you choose 'Multiple times', it is possible to go through the automation multiple times via the same trigger.
If you use the 'Subscribes to list' trigger and choose 'Multiple times', a contact will only be added to the automation again if they have unsubscribed from the list before.