WayMoreConfused

Thank you both for the very informative answers. I was a little lost for a bit. Very helpful stuff!

I looked through the forum and couldn't find quite what I'm looking for. Say I have a blue background and I want that background to only be visible through a square on the photo and the rest of the image is unchanged. I think I'm wording this right. Also, is it possible to do with text? Thank you in advance.