Layers of reflective material wrapped around the inner vessel inside the evacuated space, there to interrupt radiant heat.
A vacuum stops conduction and convection but it does not stop radiation. Heat still crosses the annulus as infrared energy from the warm outer jacket to the cold inner vessel. Multilayer insulation breaks that path: each layer reflects most of what reaches it and re-radiates the rest at a lower temperature, so the heat arriving at the inner vessel falls sharply with the number of layers.
More layers means a lower evaporation rate, and it also means more material sitting inside the vacuum, which is why the materials have to be chosen and cleaned for low outgassing. A poorly prepared wrap adds heat load in one direction and shortens vacuum life in the other.
It is one of the reasons two vessels of identical capacity can differ substantially in what they cost to run.
