Flutter is an open-source SDK developed by Google that allows developers to build mobile, desktop and web applications from a single codebase. It uses Dart as its programming language, which draws inspiration from languages like Python, Java and JavaScript. In Flutter, everything is a widget - basic visual components like text, buttons and images are widgets that make up the visual layout and interface. Common widgets include Scaffold, Column, Row and Text, which are used to build the basic structure and visible elements of a Flutter app. The document encourages developers to start coding Flutter apps on the online code editor at https://www.flutlab.io.