Mastercam 9.1 is a version of the Mastercam software that was released with new features, enhancements, and bug fixes. It provides users with a comprehensive set of tools for designing and manufacturing parts, including 2D and 3D machining, milling, turning, and more. Mastercam 9.1 supports various CNC machines and offers improved performance, making it a popular choice among machinists and manufacturers.