Passports are used as a means of identification or claiming ownership over certain aspects.
You can craft a Passport by using the Engraver
Certain blocks (like the Director, Chunkloader, Mailbox) can be protected from outside tampering by placing a passport item inside of it. Only the user that owns the passport can configure that block.
For a simple player passport, that user is the player themselves. For a guild passport, it's the guild founders.