It means that the students meet in a virtual classroom where they actually see and hear their teacher, real-time. They are also able to chat with each other and do small group work in what we call "breakout rooms." The kids, if they have mic and video capabilities, can also be seen and heard in order to present projects to the class or interact with other students or their teacher.
In addition to the live sessions, every course you sign up for has a "home-room." That is where your teacher uploads supply lists, assignments, administers quizzes, posts class recordings and even the link that will take you to each of your live class session