How is it created, and is it difficult to create an Onscroll animation?

Question

Python

Like here - scrollmagic.io
Where scrolling to a certain place starts the animation of some element

Answer 1

well , i is an iteration, and where does j come from?

Answer 2

$(window).on('scroll', function() {
    if($('html, body').scrollTop() > какое-то число пикселей от верха страницы) {
        //анимация
    }
});