Okay kiddo, a travel visa is like a special permission slip that you need to enter a different country.
Just like how you need to ask your teacher for permission to go on a field trip, you need to ask the government of the country you want to visit for permission to enter. This is called a visa.
Sometimes, you don't even need a visa at all to visit certain countries, but other times you do.
It's important to follow the rules and get all the necessary paperwork you need to travel to a different country, or else you might not be allowed to go.