Preloader is the loading screen with animated icon that you see while your page is loading until it's fully loaded by browser including all images and dynamically calculated values. Preloaders are simple and lightweight plugins that won't affect your website performance overall.
Since a lot of themes and 3rd party plugins have complex features that require dynamic values, the preloader will help to avoid issues such as jumping content or blank areas in content area of the page.
First, download the plugin, install it on your website and activate it.
Next go to Settings > Flat Preloader to select the icon:
Click Save Changes button and that's it! Now when you open any page on front end you will see animated icon until page is fully loaded.