FLTK, or the Fast, Light Toolkit, is a cross-platform C++ GUI toolkit designed for building graphical user interfaces. It is lightweight and efficient, making it suitable for applications that require a small footprint and fast performance. FLTK supports various operating systems, including Windows, macOS, and Linux, allowing developers to create applications that run seamlessly across different platforms. One of the key features of FLTK is its simplicity, which enables developers to quickly design and implement user interfaces. It provides a range of widgets, such as buttons, sliders, and text boxes, along with support for OpenGL graphics. This makes FLTK a popular choice for both beginners and experienced developers looking to create responsive and visually appealing applications.