A GUI toolkit is a collection of software tools and libraries that help developers create graphical user interfaces for applications. These toolkits provide pre-built components like buttons, text boxes, and menus, allowing programmers to design user-friendly interfaces without starting from scratch. Popular GUI toolkits include Qt, GTK, and JavaFX. Each toolkit has its own set of features and is designed for specific programming languages or platforms. By using a GUI toolkit, developers can save time and effort while ensuring their applications have a consistent and appealing look and feel.