When building or editing an Aura, you'll notice that all overlays you add in have two tick boxes, "initially hidden" and "fade-in".
While fade-in is fairly self explanatory, initially hidden could be anything!
This tick-box is to prevent an Overlay starting as soon as the Aura beings. It remains inactive & not visible until it receives a start action from another Overlay.
This is useful for when you're creating a sequenced Aura - with a whole bunch of Overlays, you don't want them all starting at once. So for an interactive menu, you'd have all your pages in the Aura with only one starting at the beginning of the Aura. Actions would then be applied to a set of button Overlays that would start and stop their corresponding pages.
For some more info on actions checkout this Actions Explained article.