Learn how to create a "parallax" scrolling effect with CSS.
Read on how to do it in this tutorial: https://www.w3schools.com/howto/howto_css_parallax.asp
Parallax scrolling is a web site trend where the background content (i.e. an image) is moved at a different speed than the foreground content while scrolling. Click on the links below to see the difference between a website with and without parallax scrolling.
Try it yourself here: https://www.w3schools.com/howto/tryit.asp?filename=tryhow_css_parallax_device