It would be a letter from a company or publisher on their letterhead explaining who you are and what and why you would be photographing a person place or thing. Usually those are given so that you can get more access or sometimes enter a place that normally would charge the public, letting you in at no charge. I photographed for a travel book and the publisher gave me a letter of assignment so that i could go into places that would let me in at no charge, that normally would charge. Its not a garantee you will get in at no charge or have special access but it helps in many cases.