A responsive site (RWD) adapts layout and sizes to the screen: on mobile, columns stack, buttons and text are large enough to tap and read; on tablet and desktop it uses the extra width.
Much of the traffic in many industries comes from mobile. A non-responsive site on a phone is awkward and often ranked lower by Google (mobile-first indexing).
“Mobile-first” design means thinking first about the small screen, then expanding for tablet and desktop. It’s a good standard for new projects.