If you were speaking to a group of people, in English one would use the pronoun "you"; in Italian one would use the pronoun "voi". Example "Do you read?" in Italian "Voi leggete?" If you are speaking about yourself and others, in English one would use the pronoun "we"; in Italian one would use the pronoun "noi". Example "We read a book." in Italian "Noi leggiamo un libro." Voi is the plural you (all of you or you all); Noi is we.
Voi siete uomini can be translated to either "you are men" or "you all are men"; voi siete ragazzi can also be translated to either "you are boys" or "you all are boys". It is important to know that "voi" is the plural you (you all) "tu" is the singular you (you the one individual I am speaking to). Does that help?
Not really as that doesn't explain why you'd use "You all are" over "You are" besides having one use uomini and the other one using uomo. With duolingo for example, voi siete uomini is only counted correct when you use you all are men where-as with ragazzi ( and i even saw it with uomo as well now that i think about it ) it counts it correct with "You are boys " but not when i go with "You all are boys". Hence the curiosity as i've heard from other italians as well that it can be translated both ways while duolingo doesn't agree with that.
Isn't using the "southern US" "y'all" in Duolingo's courses kind of silly? Who learns a language without understanding the difference between "you" in singular and plural? The phrase "You are men" clearly shows that it is the plural second person that is sought in the question.
"You all" is an attempt to translate the plural you in a language that doesn't distinguish the two forms anymore.
- Tu sei: old English "thou art" -> modern English "you are"
- Voi siete: you are
To distinguish the second meaning duolingo sometimes uses "you all", "you guys" and so on.