It's a container for ads. It sounds like you may have an ad blocker installed? Which would explain why you don't see the ad and instead just "see" the transparent bar.
Are you experienced using custom stylesheets and CSS? That's probably the simplest solution. That's what I used to use to remove the ads prior to being Premium. (And I still use it to have my own "dark mode" version of Deckbox!)
If that sounds appealing to you, but you have no idea how to go about it, then let me know what browser you're using, and I can provide step-by-step instructions to do it. It won't entail too much.