The main function of a monitor in a computer system is to display visual output generated by the computer. It acts as the primary interface between the user and the machine, showing everything from applications to videos and images. Monitors use various technologies, such as LCD or LED, to produce high-quality images and colors. Users can adjust settings like brightness and contrast for optimal display quality. Overall, the monitor is crucial for tasks such as design, gaming, and general computing, providing a platform for interaction with the system.