From the Manual
Sometimes many layouts in a project have the same content on a particular layer, such as for interface or HUD overlaid on to the game. Changing this content then becomes a chore since changes must be repeated on every layout. Global layers are aimed at solving this problem.
If a layer's Global property is set to Yes, then every layer in the project with the same name is overridden by that layer. The initial objects, as well as its properties, are used instead of the other layer's own content and properties. Then changes can be made once to the original global layer, and the changes will be applied project-wide.
The layer with the Global property set to Yes is where all changes must be made from. On other layers in the project with the same name, the Global property will be read-only and display (yes, overridden) to indicate it is being substituted by a different layer. It will also not display in the layout view on other layouts, although its contents will appear when previewing the project.