An airlock is a sealed chamber that allows people or objects to move between two environments with different pressures or atmospheres. Commonly used in spacecraft, submarines, and clean rooms, airlocks help maintain the integrity of the internal environment by preventing unwanted air or contaminants from entering. When someone enters an airlock, they first close the outer door, which allows the airlock to equalize pressure before opening the inner door. This process is crucial for safety and efficiency, especially in environments like space or underwater exploration, where sudden changes in pressure can be dangerous.