Parallax is an effect where the background content or image in this case, is moved at a different speed than the foreground content while scrolling. Check out an example below to get a better idea of it.
<div class="parallax-container"> <div class="parallax"><img src="images/parallax1.jpg"></div></div>
$(document).ready(function(){ $('.parallax').parallax();});
The parallax container height is what defines how much of the image can be seen. This is set to a default of 500px. You can add your own style to override this.
.parallax-container { height: "your height here";}