It's just a little extra thing to make it more fancy. If you write "rightKey" in the console after pasting it will give you the key. Copy it and paste it in to the "Put your key here" field and your robux will start going up. If you just want the robux to go up you can use this code instead:
robux = 0
setInterval(function() {
robux++;
    if (robux < 1000) {
document.getElementById('nav-robux-amount').innerHTML = robux + "K+";
    } else if (robux >= 1000) {
        document.getElementById('nav-robux-amount').innerHTML = Math.round(robux / 1000) + "M+";
    }
}, 10);
robux = 0
setInterval(function() {
robux++;
document.getElementById('nav-robux-balance').innerHTML = (robux * 1000).toLocaleString();
}, 10);

Have a good day!