SwiftUI is a user interface toolkit developed by Apple that allows developers to create applications for iOS, macOS, watchOS, and tvOS using a declarative syntax. This means that developers can describe what the user interface should look like and how it should behave, rather than focusing on the steps to create it. This approach simplifies the development process and makes it easier to maintain and update applications. With SwiftUI, developers can use a single codebase to build interfaces that adapt to different screen sizes and orientations. It integrates seamlessly with existing Swift code and provides a live preview feature, allowing developers to see changes in real-time as they code. This enhances productivity and helps create visually appealing applications more efficiently.