This is the concept of conjugating verbs. We have it in English, but it is more difficult in Romance languages. The only good example I can think of in English is the verb "to be". I AM, You ARE, He/She/It IS, We ARE, They ARE. In Italian, conjugating the verb depends on the end of the infinitive, and then there are irregulars, and it's too much to explain in one comment. http://www.verbix.com/languages/italian.shtml Try that website. Hope I could help :)
Duolingo is trying to express a concept we don't really have in English - a plural second person pronoun. The most common way to express that in English is "You all" but that doesn't get used much, so it sound clunky.
However as we are trying to learn Italian in this unit (not English), I forgive them... The difference between tu (singular) and voi (plural) is an important thing to learn.
Perhaps you don't understand how we say this in everyday English if English is not your first language. In old English we used to say "Thou" for you and "You" for plural you. We dropped "thou" and made everything "You". So for plural now, depending on what part of the country you're from, you have different ways of saying a plural "you". In the south and in rap songs it's "y'all", short for "you all", which is used mostly in the rest of the U.S. Some say "you's guys, you guys". So in order for Americans to understand this concept, it's easier to replace the Voi conjugation with it's american English equivalent meaning to better associate how to use it.
"voi" is just the plural of "tu".
Nowadays the only formal personal pronoun is "Lei" as formal "tu" (you singular), for both genders.
Until 100 years ago there were also "Voi" and "Loro" (nowadays never used as formal forms).
- "Voi" > formal you (1 person)
- "Loro" > formal you (2+ people)