What is a preloader? Preloaders (also known as loaders) are what you see on the webpage screen while the rest of the page's content is still loading. Loader helps to entertain the visitors or content's viewers while the rest pages are still loading. And preloader help to maintain the bounce rate of the website.
As you have seen in the Windows Screen while you switch-on of the PC/Laptop there is shown a rotating dots or loader with "please wait" text and while the loader is rotating after a few seconds, those dots are disappeared for few seconds and again appear.
In this program, this loader is the same as the windows starting loader which rotates clockwise infinitely. When this loader at 76% to 100%, this loader dots are disappeared and again these dots appear when the loader is at 40%.