You can configure automation triggers to run once or multiple times for contacts. This setting is located in the trigger "Action Options" modal:
Runs once means that contacts enter your automation through the trigger the first time they meet the trigger conditions. They cannot re-enter the automation through the same trigger.
For example, suppose your automation has a "Submits a form" trigger and is configured to run once. Contacts can enter your automation through that trigger the first time they submit your form. If they submit your form again, they will not re-enter the automation through that trigger.
Runs multiple times
Runs multiple times means that contacts enter your automation through the same trigger each time they meet your trigger conditions.
For example, suppose your automation has an "Opens/Reads an email" trigger and is configured to run multiple times. In that case, contacts can enter your automation through that trigger each time they open your email.
If you are using "Runs multiple times" with a "Subscribes to list" trigger, contacts can re-enter your automation through this trigger if they unsubscribe from your list, then re-subscribe to your list.
There is a limit of 10 entries through the same start trigger in a 24 hour period.
For example, if a contact enters an automation 10 times from 2:00pm CST to 10:00pm CST, they will be blocked from entering the automation again via the same start trigger until 10:00pm CST the next day.