A back balance is the pose where you are balanced on your lower back on the lower bar of the hoop. The easiest way to enter it is to shuffle in from seated, although in the beginning it won’t be particularly elegant. The balance point can be quite different depending on your weight distribution and the details of the pose. For me, it’s pretty low.
Initially, you’ll probably want to hold on to the hoop with your hands. As you get more confident and stable in the pose, you can slowly take them off. You can lower them below your head, which will challenge your balance. You can try different variations with the legs. Bent will probably feel easier, but it’s good to learn how to keep them straight and engaged. You can also raise on or both legs towards the top bar (both feet up is often called a pike back balance).
From here, you can easily transition into for example Star on the Bar (although balance point might not be exactly the same) or into front balance.
Here’s a video demonstrating how to learn back balance: