What is it?
Fluid Responsive Web Design & Development is an approach where all elements are sized using relative units, allowing the entire layout to scale smoothly with the viewport.
It's a convenient way for designers and developers to keep layouts visually consistent and true to the original design across screen sizes. You don't need to worry about a headline that looks fine on a large screen but breaks the layout on a smaller one.
Check out the demo - https://www.vettvangur.is/demo/#fluid-responsive-web-design-and-development - and resize the browser window to see how it works in practice. Watch how the page switches between modes and pay attention to the UI indicators - current breakpoint, media query range, viewport width, and website scale.