The term back tension, commonly used by serious archers, merely describes the use of the muscles of the back, rather than the muscles of the arms, to hold the bowstring at full draw. When you use back tension, you'll hold steadier because the back muscles are larger and stronger than the arm muscles, and because they're positioned closer to the spine, they are more stable.
When using proper back tension, you should feel as if you're trying to squeeze a dollar bill between your shoulder blades at full draw.
A good way to demonstrate back tension is to get into the full-draw position without a bow. Have someone stand in front of you and grab your elbows and try to pull them together. Try hard to resist. Since you've removed the arm muscles from the equation, the back muscles alone are preventing you from collapsing.
This demonstration also illustrates how your arms and, more importantly, your hands, can re-main relaxed during the shot.
One thing that might help you develop solid back tension is to move your release elbow slightly up and back. This transfers the holding weight from the arm to the back muscles. If you're pulling right, your shoulder blades will come together and your shirt will be loose across your back at full draw.