A travel document is like a special paper that allows people to go to different places far away from their home. Kind of like a passport or a visa.
When people want to go visit another country or even just cross the border to get to a different place, they need to have a travel document that proves they are allowed to do that. Just like how you need permission from your parents or guardians to go stay over at a friend's house for the night.
A travel document can have important information like your full name, date of birth, and even a photo of what you look like. This is to make sure that people can't pretend to be someone else to travel to a different place.
Think of it like a magic ticket that you need to have with you to be able to take a ride on a special train that can only go to a place far away. Without that ticket (or travel document), you might not be allowed to get on the train and take that exciting journey.