A passport was a document verifying the holder's identity.
Fitz Kreiner was born in 1936 and carried around a battered copy of his birth certificate. Miraculously, he convinced the British passport authorities in 2001 to issue him a passport on the basis of that document. He somehow convinces them that it reads 1976, rather than 1936. (PROSE: Escape Velocity)
As the TARDIS recharged from a scar in the Cardiff Rift, Rose invited Mickey over to Cardiff, claiming that she might need her passport if she ended up lost on Earth, an excuse Rose later admitted was just to see Mickey. (TV: Boom Town)