It's the plural of "tú" (second person singular).
Tú eres americano = You are American (singular)
Vosotros sois americanos = You are American(s) (plural)
In Spain, the words "tú" and "usted" are used when addressing people. Which one you should use depends on the situation. Their plurals are "vosotros" and "ustedes". I tend to use "usted" in formal situations or where politeness is required, or with older people. In most other cases I use "tú". When addressing a group of friends I would usually use "vosotros".