PSID is a page-scoped ID for every user in your Contacts that Facebook creates to handle different communication channels.
It's common for a business to use multiple channels to communicate with its customers, e.g., a website, a mobile app, and Messenger. When a person uses Facebook Login on a website or a mobile app, an ID is created for the specific Facebook app, which is called app-scoped ID. When a person interacts with a business via Messenger, an ID is created for the specific Page associated with the bot in Messenger, which is called Page-scoped ID. This means, for the same person, the IDs across different communication channels are different. Keep in mind, app-scoped IDs are created when a user logs on with your app, so you won't see app-scoped IDs for users that haven't installed, and signed in with, your app.
You can download PSIDs of all your contact as .csv file here:
Note that you will need PRO subscription to do that. The downloaded list will contain only PSIDs and nothing more. PSIDs are available for the Messenger Contacts only. Please, note that the Manychat Contact ID is not equal to the Facebook PSID. That list can be inserted, for example, into Facebook Ads Manager during targeting settings. Ads Manager will recognize them and you will be able to create a custom audience from your contacts.