The filter in the kettle is a scale filter. The scale filter is used to filter out the powdery whitish layer in your kettle. Lime scale deposits on the bottom of the kettle result in loss of energy and can affect the service life of the appliance. The appliance switches off before boiling if the layer of scale is too thick. It is then very difficult to remove. The appliance should be de-scaled regularly depending on the level of the water hardness.