[html] Why there is this "clear" class before footer?

I have recently been modifying a template from Blogofile, which is written in HTML 5.

When I looked at the generated page, there is this

<div class="clear"></div> 

box in the html source code before footer. What does this mean? How do I set CSS accordingly?

A class in HTML means that in order to set attributes to it in CSS, you simply need to add a period in front of it.
For example, the CSS code of that html code may be:

.clear {     height: 50px;     width: 25px; } 

Also, if you, as suggested by abiessu, are attempting to add the CSS clear: both; attribute to the div to prevent anything from floating to the left or right of this div, you can use this CSS code:

.clear {     clear: both; }