What does in terminal mean and how do I get to it?

Applications -> Utilities -> Terminal

is a terminal emulator.

It emulates a dumb terminal that was just a keyboard, a 24 line by 80 characters per line terminal that sent what was typed to the main frame computer in the glass computer room, and displayed the text send back from the computer to the display.

The interface is a simple command prompt, and the user needs to type commands to a command interpreter called a 'shell'.

If you are receiving instructions to enter some command in the terminal, then they are asking you to launch Applications -> Utilities -> Terminal, and enter the commands into the terminal emulator window.

NOTE: it is wise to either really REALLY trust the person giving your the commands to execute, or you should have a clue about what the commands are going to do for you.

In addition to the great explanation provided by @BobHarris you should make sure to have good verified working backups before doing anything in the Terminal. It is very easy to wreck macOS and delete all your data when entering commands in the Terminal app.

And NEVER just blindly copy & paste commands since there are many times where you must customize or modify the commands being provided to you. Also be very careful typing the commands since one wrong character can cause major problems.

Whenever I search for help online and I need to enter terminal commands which I am unfamiliar, I always research the commands to understand exactly what they do and how they work to make sure they will not break my system. If it breaks my system, then I may be able to reverse the changes. If I'm smart I could always restore from a backup made just before I issued the terminal commands. I'm not always smart ;-)

If you are asked to edit a file, then it is best to make a copy of the file first before you edit the file.

We don't mean to scare you away from using the Terminal, but you should have a healthy respect as to how powerful and dangerous it can be. macOS will not be protecting you when using the Terminal. Just be very careful and be prepared to restore your system from a backup.