The Contact enters list entry point automatically starts a flow whenever a contact is added to a specific list.

The new contact may have signed up through a form on your website, been imported from a file, or been manually added by you or your team. In all cases, the flow starts as soon as that contact is added to the list.
This trigger is perfect for welcoming new subscribers, starting an onboarding journey, or launching any other email sequence designed for contacts interacting with your communication for the first time.
For example, you could set up a three-message series: the first to thank them for subscribing, the second to share more about your brand, and the third to offer exclusive content.
How to configure the “Contact enters list” trigger.
1. Select the target list.
When setting the entry point, choose the list you want to monitor. The flow will trigger every time a contact is added to that list.
2. Decide whether to enable “Repeat Flow”.
By default, a contact can enter the flow only once. This means that once a contact has completed the flow, they won’t be able to repeat it.
If you enable Repeat Flow, that contact will be able to go through the sequence again.