@HelloAndrew On a Mac I use Spotlight and type Terminal. On Linux flavours there will usually by some Terminal app in menu although it might be a little deep by default). On Windows you can use the search feature of the Start menu and look for PowerShell (which has its own flavour of shell).
In That Directory
At the command prompt type “cd” (change directory) followed by a directory name to go down into that directory. Use “..” as the directory name to go up a level.
@HelloAndrew You can also use ls (list) (or sometimes dir (directory)) to list the files in the current directory.
python parser.py
This is the command to run python (a program that executes scripts written in the Python programming language) followed by the name of the script (which will be in the directory you cded into). You will need to install Python to use it.
https://missing.csail.mit.edu/2020/course-shell/ might also be useful.
Hope this helps.