I have gotten this 3 times and am wondering what it is?
Sent from my Galaxy Nexus using DroidForums
As mentioned above that is Odin mode, aka Download mode. Essentially you can flash firmware to your phone through this, updates, ROMs...quite a few different things can potentially be flashed through it. Kinda similar to .sbf if you are familiar with the Motorola line, although this is different at the same time. I am not sure if it will be useful for the SGN though.
You can get there by holding volume down while powering on.
You got to that screen because you were holding down the volume down button while powering on the phone. Freaked me out the first time I did it. LOL