That’s called a parallax background. Traditionally this would be done using an third-party plugin script like this Parallax.js | Simple Parallax Scrolling Effect with jQuery
Yet now there are CSS options, meaning we can do this directly within Webflow itself:
How to make parallax elements in Webflow
Parallax in Webflow. No Code Required
Different Parallax effects possible with Webflow (no custom code required)