The black window with white text, through which you talk straight to the computer, with no mouse in between.
Every shop has a front door customers walk through, and a back door only staff with a key use. The front door is convenient, it has a display window and everything arranged for outside eyes. The back door has none of that, just a door and a lock. The terminal is the computer's back door. The icons and buttons you usually see are the display window. The terminal is the staff entrance, used by the people who build the system itself.
When I install something on my computer that isn't an ordinary app-store program, I do it through the terminal. I type one sentence in words and the computer runs it immediately, without waiting for me to click through several windows one after another. That sentence is called a command.
A command often comes with a small addition that specifies exactly how it should run. That addition is called a flag. If the command is show me the files, the flag might say show me the hidden ones too. It's like telling the person behind the door not just what to bring, but exactly how to wrap it.
When the same sequence of commands repeats often, I write it once into a small file and then run it with a single move. That saved file is called a script. Instead of explaining every step to the person behind the door every time, you leave them a note with all the steps at once.
Professionals work through the terminal because it's faster and more precise than clicking with a mouse. The mouse is good for browsing. A sentence is good for exact work. When you need to tell the computer something specific, words get it done more reliably than a pile of menu clicks.
The hands do it on their own
I open a terminal by reflex, before I've even thought about why. The hands do it on their own, while the head is still working out what I want. That's why I know how misleading this window's look is. It looks cold and closed, but it's actually the most direct conversation I have with the machine. No menus between me and it, just words.
So I don't get annoyed when someone's startled by it. It's the shape that startles them, not the content. Once you understand that inside you're just typing sentences to the computer, the fear disappears.