A visa is a travel document that allows you to enter a foreign country for a specific period of time. In most cases, you have to apply for a visa before travelling, either at an embassy, consulate, or online. Sometimes you can also obtain a visa on arrival. Visas are usually affixed onto your passport and state how long you can stay.
Most countries impose visa requirements for foreign nationals as a security measure: to keep track of who enters and to stop illegal immigration. Visas are also used as a defensive effort, stopping security risks from entering a country.