A clone layer is a layer that keeps an up-to-date copy of another layer. You cannot draw or paint on it directly, but it can be used to create effects by applying different types of layers and masks (e.g. filter layers or masks).
For example, if you were painting a picture of some magic person and wanted to create a glow around them that was updated as you updated your charachter, you could:
As you keep painting and adding details, erasing on the first layer, Krita will automatically update the clone layer, making your "glow" apply to every change you make.