New Vegas passport
You use it to get into the New Vegas Strip. This can be obtained by completing the G.I. Blues quest for the Kings located in Freeside. On completion of the quest The King gives you a couple options for a reward, one of them is to get a free passport at Mick & Ralph's store located by Freeside's east gate, otherwise the passport is 500 caps (this option to buy the passport without using the King's favor is only possible if you can pass a speech check of 50). If you have a high enough barter skill, there is an option when talking to Ralph to lower the price to 375 caps.
However, the player can still get into the Strip as long as he or she can pass the credit check of 2000 caps. Note that you do not lose your caps, you just need to be carrying 2000 caps.
Note: A passport is not required if you enter the strip via the monorail at Camp McCarran or if you have Science above 80, in this case you can convince the Securitron to allow your entrance.