An App that has layers as part of the system opens a whole new world to experiment with…..ASKetch and other more simple apps do not have layers so are used differently.
When you are painting or drawing on real canvas or paper you always work one layer of paint on top of another. For example a second wash of water colour on top of the base colour can change the hue of a sky. The wash is applied on top of the colour underneath. Using thicker oil paint the texture of an object can be applied over a base colour and cover it. Everything goes on top!
However in the digital world Apps that have layers allow you to work in a non-linear way. Layers are transparent sheets that can be added above or below your image.
You can have a few layers or many. When painting, you can add, change or discard backgrounds without losing or altering objects in your painting, simply by moving the layers. You can experiment with brush work on a layer and keep it or discard it without losing the rest of your work. Layers add a new dimension to painting and drawing on the iPad!
When learning an App you really need to practice….a bit like learning a musical instrument…