Can't get rid of small white window on my screen

[Mary]

Hello! I would like get assistance with fixing a problem that has been going on for four days now... A small white window popped up on the bottom of my screen and I can't get rid of it... I can't click on it, I can't drag it, I can't do anything, it just stays there...
I tried restarting my computer, which made it disappear for about 1 hour, and then it popped up again...
I don't know what's causing it and what to do...