Hi Bowman,
Vosotros/Vosotras - 2nd person plural - is "you [all]." The "all" is nearly always implied in English, but on this course the teachers include it in the translations some of the time to make clear the context. So the 2nd person plural will be used when addressing or talking to a group of people (more than one) in the familiar category e.g. children, young people, your friends, and family members.
Ustedes is the 3rd person plural for "you all" and it would be used in a formal setting, for example a tour guide talking to elderly people on a coach, or a speaker addressing an audience.
Hope that helps.
John