A visa is a travel document that allows someone to leave or enter another country. They also specify how long and under what conditions you are allowed to remain in the country. When Jewish people were trying to flee Nazi Germany, they needed valid visas to leave Germany and also entry visas for the country they were entering. To obtain both visas within matching time periods (sometimes limited to just two weeks) were often incredibly challenging to acquire and expensive, which meant many were unable to leave.
