Addressable RGB LED Art Project

Hey guys,

basically im a young electronics engineer who has a passion for arty farty things and this is a project ive been brewing for sometime.

Overview:
The actual end product is probaby best described as this; Its an A3 sized sheet of clear/slightly misted plastic that has a grid of RGB LED's laid behind it, which are controlled by the Espruino.

I then produce black and white artwork onto sheets of clear acetate (causing it be black and clear artwork). This artwork is then overlaid/pinned to the item described previously. What this hopes to achieve is that it provides me with a piece of black and white artwork (sorta comic style) in which I am able to control the flow of colour within the 'white or clear' areas.

Think like what doctors pin their x-rays to, except it looks like your bones are taking a funky acid trip :)

Problem:
Currently not having any issues with the physical build, but I am having issues with the coding and general understanding of the control of the LEDs. I have little coding experience, i get what a unit array is and a for loop, and the need to define, then call a function etc.. but nothing beyond very basic work.

I was hoping someone could help me dissect the code from the RGB LED tutorial on here (this is the tutorial:http://www.espruino.com/Individually+Addressable+LEDs)

var rgb = new Uint8Array(25*3);

function getPattern() {
  for (var i=0;i<rgb.length;i+=3) {
     rgb[i  ] = i*10; 
     rgb[i+1] = i*10;
     rgb[i+2] = i*10;
  }
}

So;
for (var i=0;i<rgb.length;i+=3)
That is the three conditions within the for loop correct? first being; var i is set to 0, the loop ends when i =25? - as thats how many rows are in my array (ie LEDs), and on each loop 3 is added to the var i?

rgb[i ] = i*10;
rgb[i+1] = i*10;
rgb[i+2] = i*10;

What is this doing? I assume its setting a value of 10 to the first RGB LED, so [10,10,10]? then the next LED would be set to [20,20,20] once the loop is completed?

Basically, id like help understanding how I am controlling the lights with relation to this Unit8Array