Ah yes! :)
You website is down right now i think?
So i cant relate it to something specific on your site but i can send you in the right direction:
So the "hover" function in CSS
You can target any element on a webpage and have something occur when you move the mouse cursor over that element "hover" over it.
So what is happening in
Is they have a container that holds two boxes - an image and basically a box that holds a block color,
The image is visible but the Coloured box has an "opacity" of "0" meaning it is invisible, when the mouse cursor 'hovers' over the image, the box is given an "opacity" of "1" making it visible!
This is the basics behind the example you provided,
Let me know further