Definition
Geometry layers enable the surfaces of a model to be grouped and organized in a hierarchical structure. This facilitates editing. Geometry layers can also be used with the configuration system to hide or display surfaces in a given layer.
The 
Surface tab in the Shaper sidebar contains each of the surfaces contained in the layer you are editing.
Each layer is represented by a zone containing its name and its properties. All the operations that modify the state of a layer are applied by default to the active model that is shown highlighted.
Initially, a model has only one layer. You can create new layers via the Layer > New menu. You can then distribute the surfaces into the various layers to form logical assemblies of surfaces.