An airlock is a special room with two airtight doors that lets people move safely between places with very different air pressure or air quality. Think of it like a transitional chamber: you can't open both doors at the same time, so the room seals you in while it adjusts the air to match wherever you're going next.

Astronauts use airlocks on space stations to go outside for spacewalks. Inside the station, there's normal air to breathe. Outside in space, there's no air at all. The airlock lets astronauts depressurize (lower the air pressure) safely before opening the outer door to space. When they come back in, the airlock repressurizes so they can remove their helmets and rejoin their crewmates.

Submarines use airlocks too, letting divers enter and exit underwater. Some scientific research facilities use airlocks to keep contaminated air separate from clean air, or to maintain different temperatures in different areas.

Without airlocks, opening a door between a pressurized spacecraft and the vacuum of space would be catastrophic, as all the air would explosively rush out. The airlock's clever two-door design solves this problem, creating a safe buffer zone between two incompatible environments.