CLI stands for Command Line Interface.
A CLI is a powerful way of interacting with an operating system. One of the first CLIs was the Microsoft Disk Operating System (MS-DOS). This was the operating system for the original personal computer (PC) built in the 1980s. The early PCs had one or two floppy disk drives and a monochromatic screen, and were completely text based. As the development of the hardware increased the performance and functionality of the PC, there was a requirement to enhance the operating system.
After the introduction of MS-DOS, Microsoft developed the first Windows operating system. This provided a Graphical User Interface (GUI) with MS-DOS which had to be loaded first. Today's versions of Windows are operating systems in their own right but it is still possible to revert back to a Windows Shell to enter text-based commands to manage the computer.