If a website is responsive it means that the website's design and layout will adapt and change based on the screen size it is being viewed on. e.g. The website will look different on your computer in comparison to how it looks on an iPad or mobile.
This is really important to consider when designing a website as it impacts a user's experience and how easy it is for them to view your website content.
Some website platforms only allow you to design for 2 screen sizes (desktop and mobile), while others allow you to create a completely fluid design that will adapt to every device screen size imaginable (e.g. Webflow & Editor X).