A passport is a government-issued document you need to travel between countries. Americans must have a passport to travel out of the U.S., even if they're just going to Canada for the weekend.
A passport is a small book that includes the name, nationality, and photograph of the person it belongs to. Its many pages are stamped each time you enter a new country. When you apply for a passport, you have to prove your identity and nationality, and have your picture taken. Colloquially, passport also means something that helps you get where you want to go: "This amusement part ticket is your passport to fun!"