Webb's backplane is the large structure which holds and supports the big hexagonal mirrors of the telescope. The backplane has an important job as it must support the weight of the 21-foot (6.5 m) diameter mirror, but it also will be carrying 7,500 lbs (2400 kg) of telescope optics and instruments.
Being the "spine" of the mirror requires it to essentially be motionless while the mirrors move to see far into deep space. Imagine holding the handle of a magnifying glass to see a tiny object. If your hand shakes a lot, it will be hard to focus on the object. Just as you would have to hold the magnifying glass handle steady with your hand, the Webb backplane has to hold the telescope mirrors steady, to allow them to focus.
This structure is also designed to provide unprecedented thermal stability performance at temperatures colder than -400°F (-240°C). At these temperatures, the backplane was engineered to be steady down to 32 nanometers, which is 1/10,000 the diameter of a human hair!
Alliant Techsystems (ATK) is responsible for the development of advanced graphite composite materials mated to titanium and invar fittings and interfaces. Invar is a nickel steel alloy notable for its uniquely low changes due to thermal expansion.