A GUI toolkit is a collection of software tools and libraries that developers use to create graphical user interfaces for applications. These toolkits provide pre-built components like buttons, text boxes, and menus, making it easier to design and implement user-friendly interfaces. By using a GUI toolkit, developers can focus on the functionality of their applications without having to build every interface element from scratch. Popular examples of GUI toolkits include Qt, GTK, and JavaFX. Each toolkit has its own set of features and is designed for different programming languages and platforms. By leveraging these toolkits, developers can create visually appealing and interactive applications that enhance user experience.