Flutter SDK is Google's UI toolkit for crafting beautiful, natively compiled applications for mobile, web, and desktop from a single codebase. A responsive app lays out its UI according to the size and shape of the screen or window. This is especially necessary when the same app can run on a variety of devices, from a watch, phone, tablet, to a laptop or desktop computer. When the user resizes the window on a laptop or desktop, or changes the orientation of the phone or tablet, the app should respond by rearranging the UI accordingly.