From the point of view of human health, the optimal value of the relative humidity indoors is 40 - 60%, when the mucous membranes of the body are ideally moisturized and do not let dirt and pathogens into the body.
In summer, we can have window open all day and the humidity inside will be OK. However, if we ventilate in the winter, the relative humidity in the room will start to decrease rapidly. This is due to the fact that entering cold air holds less humidity than air at a higher room temperature. Warmer air expands more and has a greater ability to absorb moisture. For example, air at a temperature of 20°C can hold up to 5 times more water than air at a temperature of 0°C.
The devices measure the relative humidity, i.e. in percentage. If the air has a relative humidity of 60%, it means that it is filled to 60% of its capacity to hold humidity. But during winter ventilation, we let in cold air with low capacity and therefore the real humidity in the room decreases.