A global layer is a special type of layer that appears on every slide of a module. You can have as many as you like in a module, and they are useful for adding items you always want to display, such as a logo, social media link or contact phone number.
Global layers are added in their own section of the module editor [?]
The "Module Editor", also known as the "Visual Editor", is an interface used to create & modify modules.
It's through this interface you can add various types of layers to your modules, as well as animation and special FX., have their own timeline [?] The timeline is the section of the module editor dedicated to animation and controlling the stacking order of layers.
From this part of the editor animations can be added, adjusted and previewed. Additionally, layers can be selected from the timeline, and can be dragged and dropped up and down on the timeline to modify which appear in front and which behind.
It can be equally compared to both a timeline in a typical video editing application, and a layers panel in a typical graphic design application, and are assembled in much the same way as you would create a regular slide.
Global layers are grouped together and can either all appear in front of your other layers, or all behind your other layers. However, you cannot have some global layers appear in front and some behind.