Thumbnails in top right corner of imac desktop of recent apps visited
I don't know how they were enabled, but every time I visit an app, i.e. Safari, or whatever, even using the keyboard, a related thumbnail appears in the top right corner of my iMac Desktop. It's most annoying because it covers critical things in Safari, for example. I can't click on the little plus to open a new page because it's covered with one of these thumbnails. If I right-click on the thumbnail, nothing happens. If I try to drag them, they don't budge. How do I disable this annoying feature?
iMac 21.5", macOS 10.14