Humidity is a measurement of how much water is held in the air. Low humidity means that the air is very dry and does not contain a lot of water vapour. This low humidity will cause plants to lose water, an important step for plant growth and photosynthesis. Plants in grow rooms prefer cooler and drier air as this is favourable to growth and prevents mould and fungal growth. Ultimately your grow room should be between 70 to 85% humidity, though during the flowering period the humidity should be about 50%.
Install a condensing unit or dehumidifier to the room. The condensing unit actively removes moisture from the air, lowering the humidity. These units are used in damp places such as basements. You can program the unit to maintain a specific room humidity, though you will need to empty the bucket of water each day, as this is the liquid extracted from the air.
Raise the temperature of the room. The higher the temperature, the more water the air can hold, which in turn will lower the humidity of the room. Be aware that this only works within a specific range; if you raise the temperature too much it can stress your plants. Check the favourable growing temperatures for the plants and keep the temperature within the suggested range.
Spread some bowls of rock salt around the grow room. Rock salt will absorb moisture in the air which in turn lowers the humidity.
Install air ventilation if you live in an area where the outside air has a lower humidity than the air inside your grow room. In particular, ventilation during the daylight hours will greatly reduce the humidity.