Blue (selection?) box following my cursor

Hi!

I don't know what my daughter did to my computer, but there is now a blue rectangle following my cursor all over the screen. One corner is locked at 456/231 on my screen, and the other corner is wherever the cursor is. It's like I'm making a selection on my screen, but it's in all of my programs - Chrome, PowerPoint, everywhere. Can anyone tell me what she did and how to fix it? My web searches have yielded no results. I have inserted a screen shot.