Layers are a feature iof the Opus editor and are a way of organising the objects on a complex 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 objects on any other layer. For example all the menu elements of a page could be on 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.
When your project is published all layers in the editor are merged to one layer though the order of objects is maintained.
Note:
There is also a layer system in the published file, allowing you to use the animation action Move Object to Layer to animate objects in front of others. This is entirely separate to the layering in the Editor which is essentially a sophisticated grouping feature.