There are multiple way to add a customer to journey:
When Customer Performs Event
With this option, you can add user to journey when they perform a specific Event on your app/website. You can use this option if the Journey's purpose is to help users get to the next stage in their lifecycle. This could be anything like signup or purchase or starting checkout.
Example use-case: Let's say you want to send thank you email when customers make their first purchase. In this scenario, you can configure entry criteria "When Customer Performs Event" with "Purchased" event.
When Customer Enters/Exits Segment
With this option, you can add customers to journey when they leave or enter the segment. Everyday, customers will be checked for entry/exits, and customer will beadded to the journey based on the options.
Example use-case: Let's say you have a segment of VIP customers, customers who are making more than 5 purchases in a year. And when they enter the VIP segment, you want to send them an automated email or sms. In this scenario, you can configure journey entry criteria "Customer Enters/Exit" segment with given segment VIP customers.
Repeat At Fixed Time
With this option, you can add customers to journey when they leave or enter the segment. Everyday, customers will be checked for entry/exits, and customer will beadded to the journey based on the options.
Example use-case: Let's say on every Saturday you keep a webinar on how to use the product that your customers buy, you want to send the reminders on friday to all customers who purchased products in that week. For this, you can setup journey with criteria "Repeat At Fixed Time" and mention weekly repeat. Mention Friday as day of week for adding the journey and you are good to go.