Mysterious white tab with a down arrow on my desktop

I recently bought a Mac mini with Studio Display. Now there is a small white tab with a down arrow that sits about 2 inches below the menu bar in the middle of my desktop. It doesn't seem to do anything when I click on it or right-click on it. It moves out of the way up to the menu bar when I click on the wallpaper.

So what is this? What is it supposed to do? Can I get rid of it?

Screenshot of desktop attached with red arrow pointing to the tab.