I need help understanding something.

I am almost entirely new to this scene, and there’s one part in a guide I don’t understand. It’s asking that I “Open a terminal window, CD to the root of your SD card, then type python3mset9.py , and press enter.” I’ve gotten the terminal open but I don’t know what “CD to the root of your SD card” means. I’ve looked at a couple tutorials but none help with me since i’m on a chromebook and not a mac.