"Doors" here refers to something in or around the engine that can close or open, stopping or allowing the flow of air (or some other gas).
I've heard of such things in a few engines. GE9X mentions "bypass doors between the booster and high-pressure compressor which open into the airflow path to reduce the chance of ice crystals entering the core." The J58 has many doors which control the overall airflow path in or outside the core.
How do these doors maintain a good seal in the closed position? Are they always rectangular? Do they need to be perfectly airtight, and if not, how good is good enough? What type of actuator is used (hydraulic? worm gear?)? How do they withstand the high heat?
I'm especially interested in doors near the turbine because those will suffer enormous temperatures. And as far as I know there are no o-ring materials that can work at those kinds of temperatures. How do they not get stuck in either position after sitting that way for a long time in the heat?