It's a design technique that simulates a 3D view.
This is achieved by the relative displacement of objects on the same plane and a background.
Basic introduction to implement parallax effect developed with vanilla web technologies (Html5, css3, Javascript).
Project page: https://gabit690.github.io/101-parallax-effect
Repository: https://github.com/gabit690/101-parallax-effect
This project was developed by Gabit for those who can find it useful.
It's another implementation, neither the best nor the worst.