mask-image is a CSS property used for overlaying transparency or textures above text/images. It basically overlays a .png image above whatever class you specify. Things like gradual fades, gradients and dirty/grungy textures can be accomplished with masks.
If you look at the css you'll see this:
#homepage h1{
font-size: 42px;
line-height: .8em;
-webkit-mask-image: url('images/mask.png');
-o-mask-image: url('images/mask.png');
-moz-mask-image: url('images/mask.png');
mask-image: url('images/mask.png');
So the mask was intended for the homepage header (h1 = header). The CSS then clearly specifies the image URL for different web browsers (webkit=safari/mac, o=opera, moz=mozilla/firefox and the normal mask-image is for IE).
If you want to learn more about mask images or see what they can do, you can read here:
http://trentwalton.c...ask-image-text/
https://developer.mo...bkit-mask-image