A page of an Ipe document consists of one or more layers. Each object on the page belongs to a layer. There is no relationship between layers and the back-to-front ordering of objects, so the layer is really just an attribute of the object.
The layers of the current page are displayed in the layer list, at the bottom left of the Ipe window. The checkmark to the left of the layer name determines whether the layer is visible. The layer marked with a yellow background is the active layer. New objects are always created in the active layer. You can change the active layer by left-clicking on the layer name (on Windows, double-click on the layer name).
By right-clicking on a layer name, you open the layer context menu that allows you to change layer attributes, to rename layers, to delete empty layers, and to change the ordering of layers in the layer list (this ordering has no other significance).
A layer may be editable or locked. Objects can be selected and modified only if their layer is editable. Locked layer are displayed in the layer list with a pink background. You can lock and unlock layers from the layer context menu, but note that the active layer cannot be locked.
A layer may have snapping on or off—objects will behave magnetically only if their layer has snapping on. By default, snapping is on when the layer is visible, but you can choose to turn it off entirely, or to keep it on even when the layer is not visible.
Layers are also used to create pages that are displayed incrementally in a PDF viewer. Once you have distributed your objects over various layers, you can create views, which defines in what order which layers of the page are shown.