Webb's backplane is the large structure that holds and supports the big hexagonal mirrors of the telescope. The backplane has an important job as it must carry not only the 21 ft (6.5 m) diameter primary mirror plus other telescope optics but also the entire module of scientific instruments. All told, the backplane carries more than 2 1/2 tons (about 2400kg) of hardware.
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.