Page loader or customized preloader.

custom.html

<div class="loader" style="display:block" ></div>
<style  type='text/css'>
.loader{
display: none;
}

.loader {
position: fixed;
left: 0px;
top: 0px;
width: 100%;
height: 100%;
z-index: 99999999;
background:#051D40;
}

</style>
<script  type='text/javascript'>
 jQuery(document).ready(function($) { 
jQuery(function($){
jQuery('.loader').slideToggle('slow');
});
    });
 
</script>