A visa is a permit to enter a specific country. Without a visa, you might not be able to travel where you want.
While you may know Visa as the name of a credit card company, a visa is a little different: it's an official authorization to enter a country. This is different from a passport, which is a type of identification geared toward international travel. A visa is a more specific form of authorization, allowing you to travel to a certain country. A visa may only last for a certain amount of time, and different countries have different standards for granting a visa.