PSID is a page-scoped ID for every user in your Audience 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 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.
You can find PSID in every user’s profile in the Audience tab:
You can download PSIDs of all your users as .csv file here:
Note that you will need PRO subscription to do that. The downloaded list will contain only PSIDs and nothing more. 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 subscribers.