A responsive website is one that "responds" to the device that it is being viewed on.
It can change shape and size to suit screen:
A responsive website design solves the problem designers and users alike faced when the first mobile devices became popular. A responsive website is designed to respond (pud intended) to the size of the screen of whatever device it is being used on; desktops, tablets, and mobile phones.
In fact, the website you are on right now is responsive! Simply click, hold and drag the corner of the screen to change the size and shape of the browser, and see how a responsive website design reacts.
(Not suitable for all browsers.)