system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what is visa? (not the credit card)

Context: I want to ride the bike from Austria to Australia. 35.000km. 365 days. Obviously, I have to cross a certain amount of countries between. (More infos on austria2australia.com/)

So: what is a visa? I was born with Schengen, I never had to apply for a visa, and the whole concept of it is strange to me. I honestly have no glue about visa.
gpt
A Visa is a document giving you permission to enter another country. Some countries require that you get permission before entering. They offer different types of Visas, depending on how long you are staying and for what purpose. You will usually need to visit that Country's embassy in your nation to fill out the application and in most cases pay a fee to get the permission to enter.