Say you want to build a scene that performs a specific action when the temperature in a specif room drops below 20°C.
For this you need a "device state change" trigger.
Click on "New trigger", then select "Device state change":
Select the feature of the device that you want to "monitor".
For example, in our case, we will select a temperature sensor in the kitchen:
Add under which condition this trigger should run the scene.
In our case, we want the scene to be executed if "The temperature is less than 20°C":
Depending on the type of device you select in this trigger, the interface adapts. For example, if you want to run a "when the kitchen light comes on" scene, your interface will look like this: