A clearfix is a way for an element to automatically clear after itself, so that you don't need to add additional markup.
.clearfix:after {
content: " "; /* Older browser do not support empty content */
visibility: hidden;
display: block;
height: 0;
clear: both;
}
.cleaner {
clear: both;
}
Normally you would need to do something as follows:
<div style="float: left;">Sidebar</div>
<div class="cleaner"></div> <!-- Clear the float -->
With clearfix, you only need to
<div style="float: left;" class="clearfix">Sidebar</div>
<!-- No Clearing div! -->