That tan, cardboard-like rectangle inside your microwave is a mica plate, also known as the waveguide cover, and it is an essential part of the appliance.
It allows microwave energy to enter the cooking chamber while protecting sensitive internal components from food splashes and grease.
Never remove it permanently or replace it with ordinary cardboard, as this can cause dangerous sparking or even a fire.
If the plate is burnt, cracked, or damaged, stop using the microwave and replace it with a compatible mica cover before using it again.
