Focus groups are an activity where the participants are divided in groups that are then involved in a moderated discussion.
Unlike interviews, there are multiple participants involved in the same session.
This can help to fuel discussions and exchange among different users and let them share and confront ideas and opinions while eventually forming new ones.
Those discussions are an open conversation on the scope, use and utility of the prototype. However, the moderator still drives some parts of the discussion and has some pre-defined guidelines at hand