I hope this might clarify what someone might mean when they ask you to send a command to the Pi.
Your Octoprint Server is still running an operating system, Rasbian if I am not mistaken which out of the box, has the camera port on the Pi disabled. You need to access the Pi via a terminal to enable this but that is not possible by normal means because we usually communicate with our Pi's in what is called "Headless mode" or in layman's terms, not having a Monitor, KB and Mouse connected.
So here is the magic, you need to access the Pi's terminal like I said before but through your network. This is done with SSH software like Putty in Windows. This will allow you to simulate a terminal window but you will be doing so over Ethernet Vs. conventional physical means.
To answer your question on raspi-config, you first need to download Putty and know what IP address your Pi is (Same as you type to get to octopi interface). When you enter your IP into putty and connect for the first time it will ask you to accept the security cert., press yes. It will then ask you for your username (for the Pi) it is default as "pi" without the "" of course. Next it will ask for a password which is "raspberry" by default, again no "". It might ask you to change that if you would like, I would highly recommend to do so but remember it as you will need it again soon (The password can be changed in raspi-config as well). Now you should be presented with a the opportunity to type "sudo raspi-config" and hit enter (sudo password will be asked and is the same as before). The screen will change to an old looking blue background with grey dialog box. Look for "interfacing options", use the arrow keys and enter key to get to it. You will see the next screen show camera interface, hit enter and yes to enable it. Now exit the raspi-config and reboot the Pi, you should see the camera if the ribbon cable has been connected correctly of course.
Raspi-cam has no LED's so unfortunately trial and error will be your friend for the day.
Please Youtube Putty and raspi-config for more info if you need it.