Layers are a way of organising a page in your publication so that it is easier to work on separate elements without affecting anything else.
Layers can be thought of a sheets of glass on which objects can be placed and manipulated independently of object on any other layer. For example all the menu elements of a page could be in one layer and the background details in another. It is then simple to hide the menu layer and work on the background without the menu items getting in the way.
To move an object up or down within a layer use the Organiser tree to drag them or the Ordering sub-menu on the main editor Arrange menu and the right-click menu for page objects.
Layers are an editor feature allowing you to have subsets of objects to make editing complex pages easier. You can also change the ordering of objects in your publication using the Move Object to Layer action in the Animation category or using the SetLayer script action. These actions will adjust the position of objects according to the layers used in the editor BUT can also be used to ensure objects move in front of or behind other objects even if no layers have been used in the editor.