Parallax scrolling is a web design trend popularized in 2014.
Parallax scrolling is a scrolling technique in which background images move more slowly than images in the foreground.
The intent of parallax scrolling is to give an impression of depth.
Parallax scrolling has been used for a long times for video games and is now sometimes used for website design and mobile app design.
Clear explanations and examples of parallax scrolling for web design :