Responsive design means a website adapts to different screen sizes instead of pretending every visitor has the same monitor.
Layouts, text, images, menus and controls need to remain usable on phones, tablets, laptops and larger screens.
In plain English
This belongs to the machinery or experience of a website. You do not need to know how to code it. You need to understand what changes for the visitor, the editor or the person maintaining the site: clarity, speed, accessibility, stability or how easy the site is to change later.
A real example
Three columns on a desktop may become one clean column on a phone, with buttons large enough to tap comfortably.
What matters is not whether the technology sounds advanced, but whether the result is felt as a clearer, faster, safer or easier website.
Why it matters
A large share of visitors may see your site on a phone. A desktop-perfect page that collapses on mobile is not finished.
Technical choices eventually become human consequences. A setting that sounds obscure can decide whether a page loads, whether someone using a screen reader can use it, or whether a future redesign becomes simple or painful. That is the level at which the term becomes useful.
When do you actually need to care?
You need to care when the site is slow, difficult to use, hard to edit, inaccessible, unstable, expensive to maintain or becoming dependent on one fragile technical choice. You also need to care before migrations, redesigns and new integrations, because small technical decisions can become large constraints later.
Useful questions to ask
- What will a visitor or editor notice if this is done well?
- Who is responsible for maintaining it after launch?
- What happens if the technology stops working or needs to be replaced?
What should you do with this?
Ask what changes for the visitor, who is responsible for maintaining it, and what happens if it fails. If nobody can answer those three questions, the implementation is probably more complicated than it needs to be.
What this does not mean
Responsive does not mean everything simply shrinks smaller.
The short version
The layout should adapt; the person should not have to fight it.
Related: mobile-first and UX. You can also read about website strategy.
Further reading: web.dev on responsive design.