It may be as simple as an airtight and watertight enclosure, or it may use active means to remove water vapor from the air trapped inside.
Dry boxes are used to safely store items that would otherwise be damaged or adversely affected by excessive humidity, such as cameras and lenses (to prevent fungal growth), 3D printing filament (to prevent moisture caused damages such as popping and sizzling when passing thru the hotend and turning into steam.
They are also used in the storage of surface mount electronic components prior to circuit board assembly, to prevent water absorption that could flash into steam during reflow soldering, destroying the part.
More sophisticated designs link the cooler to a settable digital hygrometer, allowing very precise humidity level control.
Another form of electronic dry box technology utilizes multi-porous molecular sieve desiccants to absorb moisture.