Microwaves in your appliance are created by the magneton, and then they travel into the cavity of your appliance through a hole on the inside wall. This cover goes over that hole and acts as a filter, it allows the microwaves to pass through it and into your appliance to heat food. It also stops moisture and food particles from getting into the mechanisms of the microwave. The waveguide is found inside the microwave, attached by one or more screws to the right side or in the roof. If you notice that your waveguide has fallen off, has a hole in it, or is sparking or arcing during the cooking process, you will need to replace the part. A new cover is rigid and firm, but over time it becomes soft and flexible due to absorbing moisture.
NOTE: Remember to disconnect the power and discharge the capacitor from the microwave before starting this repair. The capacitor can hold a charge even when power is disconnected from the unit.