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.
A high humidity can lead to mold developing. The best way to prevent this is to ventilate your home when humidity levels are too high.
You can learn more about relative humidity on Wikipedia.