Artboard's layers are discrete, like a stack of tracing paper (or transparent mylar). Just like drawing on paper, each layer can hold multiple graphics in your drawing. You can use each layer as its own "page" to organize your graphics. Within each layer, drawing objects have their own stack order and objects can be moved in front of or behind each other. Just like with a stack of tracing paper, your graphics on upper layers are shown “drawn on top” of objects in lower layers. Create your drawing with one or more drawing layers. Use layers to stack objects in front of or behind each other, to organize for printing or exporting, and to show, hide or isolate objects for easy editing.
The Layers panel lists all layers in a drawing. The layer that you are working on is highlighted for easy identification. So it's a good idea always to check which layer is active. For instance, if you try to choose an object and nothing happens, check whether you are in the right layer.
A Guide layer and Graph Paper layer are available in every drawing file. Click the "show" (eye) icon to show (or hide) them.
To work with layers, you should have the Layers panel visible. Using the Layers panel in Artboard, you can select layers, create new layers, remove layers, show and hide layers, arrange the stack order of layers, and create layer masks.