What does backdrop do?

What is the Backdrop setting for? Looking at the WeWeb Sidemenu element, and there are display condition for Backdrop on Tablet size and Mobile size. I don’t understand what a Backdrop is…
Screenshot 2024-04-04 at 1.44.05 PM

I would assume it’s for “backdrop filters” - backdrop-filter - CSS: Cascading Style Sheets | MDN ??

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thanks