What is the "splash thing" on my site's main page?

Site URL: http://www.katadair.com

Over a year ago, I set up a little "splash screen" that would prompt visitors to my site to enter their information so I could notify them when an online course launched.  I have no idea what to call it, and I have no idea where to turn it off or modify it.  

It doesn't show up when I go to the site, but I know it shows up when other people do.  If you go to my site, you'll see it.  

I am launching the course Monday, and I'm trying to convert my site...I need to remove this...urgently.

I appreciate your help!  www.katadair.com