Instructions instead of buttons
A graphical interface offers buttons and menus; a CLI lets you type instructions instead. For example, you might type a command to ask it to list the files in a folder, specifying which folder to check.
A terminal is a text window for entering CLI commands and seeing their results. Developers use it to manage files, run builds, work with version control, and interact with cloud services. They can put commands in a script to repeat them automatically, including in CI/CD.
Commands can run on your computer, a remote server reached through SSH, or a CI runner. They may also send requests to cloud services that run elsewhere.
Powerful and unforgiving
CLIs let you work quickly, combine commands, and document exact steps. A mistyped or copied command can have wide effects without asking for confirmation. Clear help text and safe defaults reduce that risk.