A Hermetic Door is a device used by Metro stations to allow, or deny, access to their stations from the surface. All doors in the Metro can be hermetically sealed, but the ones directly referred to as hermetic doors are always the entrances to the Metro.
In the real world and the world before the apocalypse, hermetic doors were built directly into the Metro when it was built. They were generally made of thick steel and concrete and were shut electronically, though could be manually shut if there was no power. Generally, they were always open to allow public access into the Metro, but in the event of an emergency, such as nuclear war, they could be sealed shut for protection. The Metro tunnels also have large hermetic doors that can be used to isolate individual tunnels.
As they are hermetic doors, when properly shut, they create an airtight seal, preventing radioactive, biological and chemical contaminants from entering the area, fulfilling their secondary use as bomb-shelter doors.
In the Metro Universe, the doors were finally used for their original purpose - to seal the Metro. When the air-raid sirens sounded, thousands fled to the Metro for shelter. As seen in the live action trailer, once the quota for people was met, the doors were sealed permanently, leaving millions of Russians on the outside to face the bombs.
Ever since the end of the world, many stations keep their hermetic doors permanently sealed, to stop anything, or anyone, from the surface into their station. They generally serve their purpose well and nothing can breach them. Other stations that regularly have Stalkers passing through, like Polis and Theatre, keep theirs unlocked, though closed when not in use, and always heavily guarded, as seen by the two flamethrower troopers at the end of Echoes.
However, despite the sealing of the hermetic doors in occupied stations, mutants have found their way into the Metro through unlocked, missing or damaged doors. The most notable of these is the hermetic door of Botanicheskiy Sad, the closest station to the Botanical Gardens, which was opened by Artyom and two friends, Vitali and Eugine, when they were children, eventually allowing the Dark Ones access to VDNKh and the Metro as a whole. Artyom keeps this to himself, for he knows he would be executed for allowing the Dark Ones into the Metro, although Hunter knows the secret, but does not tell anyone.
- Paveletskaya is another station that is missing its hermetic door - it was taken for scrap not long after the war, and there is no way to replace it. As such, every night, the station's residents hide in the tunnels while the guards stay awake to guard the escalators to the entrance. No station dares to attack them as they effectively hold an entrance for mutants to flood the Metro if they fail to guard it. But in real life, Paveletskaya have hermeteic doors - https://vk.com/video219723850_456239598
- Hermetic doors are sometimes used to isolate stations that have the plague. Hansa have done this for the three plague stricken stations beyond Marksistskaya, until a cure can be found, though it is likely the station will just be cleared with flamethrowers like other diseased stations.
- Many stations will only open their doors to let people in from the outside if they know the correct knock, such as the secret knock Artyom has to remember to let him access Smolenskaya, though in his haste to escape the watchers, he forgets, but the door opens anyway.