It is a document given by a foreign country to you to visit and enter their land. The visa must specify the reason for the visit and how long you’re going to stay ther
A passport might sound like a visa, but this document is issued by your home government to let you go abroad. It proves the nationality of the country you belong to.
Your passport is issued by your home government. It acts as an identification for you in international waters. Your visa is issued by the embassy of the country you’re visiting.
Being a US citizen, you plan on taking a vacation in Iceland. Since there are no restrictions in the country to arrive without a visa, you can easily visit here without a problem. However, you’d need your passport that would grant you entry and exit from Iceland. Keep in mind that Iceland only allows you to stay as a US citizen in the country for only 90 days (about 3 months).