I know it's a stupid title but I have no way to Google this because I don't know how it's called.
Basically if you go to Skrill's website https://www.skrill.com/en/merchants/ and you scroll down, the background itself stays, while the elements are transparent on top of it.
How is this called and what are some CSS rules I can look into to do this?
If you are in Chrome, you can 'inspect' (in right-click menu) while on a website to see what the CSS properites are. The website you reference has a div with the class full-background-container and inside that is a div with the background image and it's positioned fixed. What you are seeing on that website is "parallax" - the movement is CSS + JS positioning during scroll events.