One way to think of a multi-dimensional array, is as an array of arrays. When you use brackets to access your array, the first set of bracket refers to the entries in the outer-most array, and each subsequent level of brackets refers to the next level of entries inside.

var arr = [
    [1,2,3],
    [4,5,6],
    [7,8,9],
    [[10,11,12], 13, 14]
];
arr[0]; // equals [1,2,3]
arr[1][2]; // equals 6
arr[3][0][1]; // equals 11