ScrOG stands for "Screen of Green". It is a method in which you set a screen above your plants (The height depends on your strain of choice... A full indica will require a screen set at as little as a foot, some people give even less. A full Sativa will require a little bit more headroom to get the branching you want. I'd say closer to 2 feet would be good) in order to limit the height of your plants, as well as encourage an even canopy to ensure even light penetration. You will be able to keep your plants very short with this method. Try to train the growth into the screen until flowering, and then let those buds swell up over the screen. For more information, look to billybob's thread,
https://www.rollitup.org/general-marijuana-growing/41321-i-call-billybob-scrog.html
he is a man who is doing it very well. Don't follow my example (my grow journal), as I [consciously] made some bad decisions. Mine, however, will show you how little vertical room you need. My space is under 4' high, and I put a 400W HPS in it. Good luck!
S-Town