You put on your space suit, and go out through that door. When you come back, the door closes, and they pump air back into the room through there.
11 seconds sound clip from the Santa Claus Conquers the Martians (1964) movie soundboard.
You can hear this line at 00:46:39 in the Blu-ray version of the movie.
Quote context
[...]
- Why, Santa makes everyone feel good, even me.
- Now, now, Billy boy. That's not the Christmas spirit.
- Why, of course. We'd love to take the grand tour. All right, children.
- Airlock.
- Sure. This is where you come when you're ready to go out in space. It's airtight.
- When it reaches the pressure of the rest of the ship, you can take your space suit off.
- Well, where's the control that opens the door?
- Not here. That's in the control deck.
- You see, once you pull that switch, the warning bell sounds, and in 60 seconds, that door opens.
- That's to give the spaceman a chance to make a final check on their equipment.
[...]