The answer to 'what does dehumidifier do' is a dehumidifier is an appliance that removes excess moisture from the air in a room or home. It works by drawing in humid air, passing it over cold coils that condense the moisture, and then expelling the drier air back into the room.
Dehumidifiers are helpful in reducing indoor humidity levels, which can prevent mold and mildew growth, reduce dust mites, and make the air feel more comfortable. They are especially useful in damp basements, bathrooms, or other areas prone to high humidity.