While this wording is vague, what this means is that the rain sensor is not currently filled with rain. If the sensor icon is red, it means rain is currently inside the sensor and the sensor is engaged which prevents the system from watering. If the sensor icon is green, it means the sensor detects no rain and is not preventing the system from watering.
Note that the sensor is meant to stay active (red) for a while after the rain has left the area and this is exactly what we want. The sensor does not report active rainfall, it reports moisture and wet conditions on site that are associated with recent rainfall.