Understanding Prototypes in javascript

I have been trying to figure this video out. I have watched it several times and have found it extremely confusing. So, I have looked online for help and, would like to know if this is what is actually happening...

function Dice(sides) {
  this.sides = sides;
}

Dice.prototype.roll = function () {
    var randomNumber = Math.floor(Math.random() * this.sides) + 1;
    return randomNumber;

var dice = new Dice(6);
var dice10 = new Dice(10);
}

From what I can gather, though this looks confusing, what we are actually doing here is simply adding a new variable into the function Dice. Meaning, there is a function called Dice with a single variable of sides. By having a prototype, we are actually saying the function dice now contains a second variable called roll.

So, we could do something similar like...

Dice.prototype.material = "plastic";

and we are making another variable in the Dice function to describe the material of the die.

Is this correct or am I still misunderstanding?