Command Line Interface is a way of interacting with the system and software by typing text commands into a terminal.
"Under Linux there are GUIs (graphical user interfaces), where you can point and click and drag, and hopefully get work done without first reading lots of documentation. The traditional Unix environment is a CLI (command line interface), where you type commands to tell the computer what to do. That is faster and more powerful, but requires finding out what the commands are."
from man intro(1)
Some users prefer using the command line for some tasks because it saves them time and effort. They also benefit from being able to create scripts to automate common tasks, and they can use the command line to configure their operating system on a very low level to exactly fit their needs.