tado° devices measure the relative humidity in your home. The readings in your home are shown in the apps in percent next to a water droplet symbol.
The relative humidity provides an indication on how much water vapour is in the air - the higher the percentage, the more water vapour, where 100% is the maximum the air could hold at its current temperature.
Lower humidity levels (below 30-35%) can lead to dry noses and a scratchy feeling in your throat.
High humidity can lead to mold developing. The best way to prevent this is to ventilate your home when humidity levels are too high.