Parallax scrolling is a visual effect used on some websites to add the illusion of depth. The effect occurs when you scroll down a page and a background image moves more slowly off the screen than the text or graphics overlaid on top of it.
This effect has long been used in animation and video games but has only been achievable on websites since 2011 using HTML5 and CSS3 coding.
It can be argued that parallax scrolling is a way to make a webpage more interesting and engaging. In certain contexts where the website is about playfulness and fun, the effect can be used to reinforce this messaging if the technique is used in a subtle (not overwhelming) way.
There are drawbacks: certain people will experience motion sickness as a result of parallax scrolling and it adds significantly to the loading time.
Research has found that parallax scrolling does not improve overall user experience and preference.