Can anyone tell me how this works? (transparent drop-down menu)

Hello all, it's been a while since my last post. Anyway, on my travels, I bumped into a website that had a really nifty navigation bar. I would like to learn this technique, but after a lot of Google searches, I'm still clueless.
So I don't get accused of site pimping (I don't have anything to do with the site in question, or know the webmaster) I will just screenshot the navigation itself and if anymore info is needed you can click here.

I know it runs on Javascript somehow, but I'm needing a clearer idea of how it works.

Here's the screenshot, transparent drop down box.

Thanks in advance :]